Inko is Izuku's mother, and she cares deeply for Izuku; when her son finds out he’s Quirkless and therefore can’t be a hero, she apologizes profusely as if it were her fault. It is hinted she takes his sadness on herself, and has gained more weight as a result, but it is unknown if this really true.
However, once he receives his new Quirk, One For All, and gets into his dream school, U.A. High School, she is happy that he got powers but feels guilty about her former beliefs, and afterwards she tries to make it up to him and support him by making a costume for Izuku, using the design of sketch she saw him make. She brings up her past actions and apologizes for not believing that he could be a hero. She soon tells him that she will support him all the way until he accomplishes his dream, as Izuku is touched at this and thanks her for her words.
She later shows that she is worried about the consequences of his new Quirk after seeing the damage done to his body after he used it. Indeed, after the ordeal at the training camp, Inko is completely against him resuming his studies at U.A. due to the staff’s inability to properly protect her son. Izuku is shocked at this and can’t say much to his mother's firm stance. Only after Izuku shows her Kota Izumi’s letter, as a sign that he’s truly on the path to being a hero does she change her mind about U.A. She also makes All Might promise to take care of him, and stay alive for him.
While upgrading his costume, Izuku only adds a few things on it and refuses to modify it even further as he is attached to it. He appreciates that his mother made it for him and he wants to keep it as a reminder that she is supporting him.
As a child and even now, Izuku admires All Might and wants to be a hero just like him. In their first meeting, All Might saves Izuku from the Sludge Villain and signs a happy Izuku's notebook. However, Izuku is very shocked to see All Might's true form and is dismayed when the latter tells him being powerless didn’t fare well at all for his chances at becoming a hero. Despite this, Izuku earns All Might's respect after he runs toward the slime monster in an attempt to save Katsuki Bakugo, and a mentor-student relationship blooms between the two as All Might chooses him as his successor. While All Might's training on Izuku is rough, the latter continues to give it his all to achieve his dream. They grow closer as All Might listens to his resolve of training and doing more than he has to become a great hero, impressing the hero.
All Might is Izuku's confidante, mentor and friend in and out of school, and they share a mutual respect for each other's heroism ideals. All Might returns Izuku’s admiration by telling him that he was formerly Quirkless before he got One For All. All Might is also impressed by how much Izuku has improved his Quirk after his time with Gran Torino, and deems it is time to tell Izuku of their Quirk's history.
Izuku's admiration and respect for All Might is so great that he avoids trying to attack All Might during the First Term Final Exam, but when it turns out he has no choice but to attack him with Katsuki's Grenadier Bracer, Izuku apologizes in advance.
At first, Izuku is an extreme fanatic who tries to imitate All Might from his style of name and costume design. As he matures later on, Izuku slowly learns to become his own hero and develop his own style. This change is noticed by All Might who is proud to see Izuku on his way of becoming his own hero and improving in his skill.
Her scolding finally reaches a tipping point, where she flat out says she will no longer heal his self-inflicted Quirk injuries after the U.A. Sports Festival, fearing that he will continue to cause serious harm to himself as long as he thinks he can just be healed by her.
Shota is Izuku's homeroom teacher. Initially, Izuku doesn’t recognize him and was put off by his deadpan manner. This lasted until the latter uses his Erasure Quirk to restrain him, which causes Izuku to remember he is Eraser Head, a pro-hero that he respects for his power and capabilities. At first, Shota firmly does not believe Izuku can be a hero due to his reckless behavior when using his Quirk, as he would be useless in battle if he kept getting injured. However, Izuku proves to Shota that he has potential and the latter is impressed and decides not expel him.
Throughout the series, their relationship can be defined as a rocky, troublesome student-teacher relationship. As his teacher, Izuku listens to Shota's orders and advice in order to improve himself. He has some level of respect towards Shota, as he is surprised at the latter handling several villains by himself. However, Shota's cold and logical personality conflicts with Izuku's personality and mindset, making it difficult for the two to get along. Out of all his students (besides Katsuki), Shota appears to antagonize Izuku the most whenever he does something that upsets him, a recurring problem being Izuku overusing his Quirk and breaking his body. As such, Shota refers Izuku as the 'Problem Child'.
However, Shota is still Izuku's teacher and as such, ultimately cares for him. He offers some level of consolation and guidance for Izuku when he is upset, and he will make sure Izuku does not do anything dangerous and act upon it. In return, Izuku once tries to help Shota when he decides to fight off several members of the League of Villains and is amazed by his abilities, since he realizes he greatly underestimated his teacher. However, he is horrified to watch him being overpowered by Nomu at the U.S.J. This shows the more caring and inspirational side of Shota helps Izuku get along better with him. Izuku also carries his teacher to safety after All Might appears.
It is possible another reason they sometimes do not get along well is because of Izuku's personality and mindset bearing similarity to All Might's, who Izuku looks up to. Shota and All Might do not get along, and this is extremely evident in Shota and Izuku’s relationship.
Recently, Shota has taken note of Izuku's Quirk gradually changing and asking him on the phenomenon and gets a vague answer.
Izuku has formed positive relationships with most of Class 1-A due to his kindness, bravery and intelligence. As seen in the provisional license exam, his emerging leadership skills play a key role in his class’ victory against the other schools. This also shows that most all of his classmates are willing to listen and follow his lead, even those he casually knows respect him enough to follow him. It has been shown that part of Katsuki’s anger toward Izuku is jealousy over how popular he is with the rest of the class.
Izuku and Katsuki know each other since early childhood, but their relationship appears to deteriorate over the years (though interestingly enough, Izuku returns the favor by calling Katsuki his childhood nickname, "Kacchan"). Ever since he fails to develop a Quirk as a child while Katsuki does, he has seen Izuku as weak and bullies him, giving him the derogatory nickname “Deku”, which he always calls him.
Izuku later surprises Katsuki by trying to save him from the Sludge Villain. Much later, Katsuki catches up with him and berates him for his actions, calling him a Quirkless failure most likely out of pride. Izuku's heroic act is also what causes All Might to choose him as his successor of One For All, thus, ironically, as he did not want this to happen, making Katsuki indirectly responsible for his acceptance into U.A.
Izuku stands up to Katsuki and enrolls in U.A. High School. He also reveals his new Quirk, with Katsuki reacting in surprise as the latter angrily questions if Izuku intentionally hid his powers from him. In a training battle, Katsuki attacks Izuku, but once again Izuku surprises Katsuki by revealing he has been memorizing his moves and knows how to counter them, as he once possesses a strategy book that details all of Katsuki and other's moves, before Katsuki burns it. Finally standing his ground against Katsuki, Izuku tells him that he is no longer the same kid Katsuki used to bully, even stating he will use the nickname Katsuki gave him as inspiration. Enraged by Izuku's bold claims, Katsuki attacks relentlessly to get him to use his Quirk, which works but ends with Izuku defeating him for once. After the exercise, Izuku catches up to Katsuki and tells him he obtained his powers from someone else, but Katsuki does not believe it and swears to defeat him.
Izuku states that Katsuki makes all of Izuku's insecurities about himself bubble to the surface. Despite the two having such a strained relationship, Izuku still admires him for his strong Quirk, natural combat capabilities, confidence, and his ability to win. Ever since the end of the Battle Trial Arc, Izuku has not spoken to Katsuki on friendly terms due to Katsuki considering Izuku his arch-rival. Izuku is also surprised to see Katsuki is the one being teased by their class, which is complete opposite to their middle school days. Even still, Izuku maintains respect for Katsuki and is shown to care for him, as shown when he tried to ensure Katsuki is safe from the villain attack, but Katsuki is then captured by the Vanguard Action Squad, so Izuku uses all his strength in an attempt to save him and spirals into despair after he fails to do so. Directly after, Izuku teams up with Tenya Iida, Eijiro Kirishima, Shoto Todoroki and Momo Yaoyorozu in an attempt to rescue Katsuki from the League of Villains' hideout.
Back at U.A. sometime later, Katsuki remarks to Izuku that he is really making his power his, revealing he believes Izuku's story of possessing a borrowed Quirk, which worries Izuku.
Later on, Katsuki and Izuku take out their stress on each other in an emotional fight, which Katsuki wins. After the fight, Katsuki learns the full truth of Izuku's Quirk and his connection to All Might, which he seems to understand and decides to keep the secret to himself.
Since the fight, the two better understand each other and have become more reminiscent to that of a proper "friendly" rivalry, although they still have a long way to go.
Ochaco is the first person that Izuku interacted with when he first came to U.A.. He saves her from being attacked by giant robot and she returns the favor by ensuring he lands safely on the ground after jumping to her rescue. The two meet again after becoming classmates and soon become close friends; they get along very well and their discussions are always bright and friendly. A running gag is Izuku blushing and getting flustered whenever Ochaco happens to be too close to him. Ochaco is the one that turns his once demeaning nickname "Deku" into a positive identity for him, when she says it gives her a "never gives up“ vibe. They share similar ways of quick thinking, which contributes to their friendship.
They mutually respect each other. Ochaco is inspired by him, striving for his level of tenacity and perseverance. They are protective of one another as Ochaco quickly calls the police after she finds Izuku threatened by Tomura Shigaraki. Izuku thinks of her as a brave and resourceful person by herself, and considers her a person very capable of devising strategic plans on her own. He even defends her from Katsuki when he says she’s incapable of fighting on her own.
Another example that he is confident in Ochaco is seen during the Provisional Hero License Exam. Himiko Toga (disguised as Camie Utsushimi) used her powers to impersonate Ochaco, but Izuku sees right through her disguise because he knows that the real Ochaco is a skilled heroine and would never jump into danger without a plan, which is what Himiko does. When Himiko sees Ochaco, she tells her that Izuku trusts her a lot.
Earlier on in the story Izuku is not able to hold a conversation with her as he gets embarrassed whenever he speaks to a girl, but as he gets to know her more he becomes able to speak confidently with her.
It has been confirmed that Ochaco has feelings for Izuku, but Izuku has yet to reciprocate those feelings, although it's hinted that he does. For example, he is very worried about how wounded and hurt she was after he loses control of Blackwhip, and later protected her from Hitoshi Shinso's Cloth before she can react.. After the battle, Mina teases both Izuku and Ochaco, which causes both to turn bright red in embarrassment.
In their first meeting, Tenya labeled Izuku's muttering about heroes as a distraction and thought he would be a hindrance to the school which earned Izuku bad attention from his peers. However, in the entrance tests, Izuku surprised Tenya by selflessly going back to save Ochaco Uraraka and using his Quirk to destroy a giant robot in the process.
When they met again in class, Izuku first thought that Tenya was scary and that they wouldn't get along, but was surprised as the latter apologized for his previous treatment to him and even considered Izuku better than him in many ways after the entrance course, much to Izuku's shock. Izuku got to know Tenya better, where he found out that while the latter was serious he was also kind and they ended up becoming friends. Izuku and Tenya are usually friendly with each other. However, sometimes their friendship slightly falters such as when Izuku criticized Tenya for seeking revenge against Stain and Tenya punching Izuku in the face for acting recklessly by attempting to save Katsuki.
Despite this, Izuku and Tenya's friendship has rebounded back many times despite these mishaps, mainly because they are only concerned for each other's well being. They are usually seen together having friendly discussions. Izuku resigned as Class President and named Tenya as his successor, knowing that he has better leadership qualities than him. After Tenya is defeated by Shoto, Izuku is saddened to see his friend lose. Since Izuku knows that Tenya has a grudge against Stain, he tries to save him from his death. While recuperating at Hosu General Hospital, both Izuku and Tenya resolve to become stronger together.
Shoto originally sees Izuku as his arch-rival, due to him having connections with All Might (a person with whom his father has a major rivalry with). Izuku, on the other hand, has a lot of respect for Shoto, admitting that he is far stronger than him. Shoto confronts Izuku at the Sports Festival, asking if he is All Might's secret love child, but the latter denies this as Shoto decides to reveal his family history. After hearing Shoto's tragic backstory and seeing his sadness, Izuku starts to show sympathy for the pain he went through and resolves to help him overcome his dark past by getting him to use his power, telling Shoto that the power is his own and not his father‘s. Although Izuku loses to Shoto while trying to help him, Izuku's actions do have an impact as Shoto starts reacquainting himself with parts of his past he had forgotten and stops rejecting his power. Shoto also recognizes that Izuku knowingly insisted on helping him, despite knowing it would lead to Shoto's victory.
Shoto even comes to his aid when Izuku and Tenya are attacked by Stain, telling his father he was going to "help a friend" when he received Izuku's location. They work well together, going on to defeat the infamous hero killer. After the battle, Izuku and Shoto are on friendly terms with each other, with Izuku even laughing at Shoto's expense when he was worried that he was cursed to always hurt people's hands. They are good friends thereafter — Shoto, who was previously shown to have little interactions with his classmates, starts to hang around with Izuku and his friends more frequently, even joining their lunch table.
Tsuyu respects Izuku for his heroic actions, as well as his victory against Katsuki. She becomes friendly with Izuku, requesting he call her by her first name, which is something he frequently forgets or is too shy and polite to do, so she corrects him. He sometimes remembers and forces himself to call her by her name to make her happy. She even notices the similarities between Izuku and All Might's powers, which worried him so he has to give her a vague answer.
During the U.S.J. Arc, she saves him from being attacked underwater and they work together well against their enemies. In turn, Izuku also tries to save her from being attacked by Tomura but fails, though they are both saved by Shota and All Might in the end.
In the Rescue Trial Arc, she is receptive of Izuku's plan to subdue the swimming villains, showing not only respect but also trust. After Shoto defeats Izuku in the Sports Festival, Tsuyu is one of the few students who go to Recovery Girl's office to see if he is okay.
Tsuyu also implores Izuku not to join in Eijiro's plan to rescue Katsuki, since the recklessness could make them no different from villains. After the rescue succeeds, Tsuyu apologizes for harshly doubting their plan to save Katsuki, as they apologize for worrying her and they manage mend their friendship.
Their friendship is built on a mutual respect towards each other. They team up along with Ochaco and Mei Hatsume during the Cavalry Battle. Fumikage respects Izuku as a leader, as he leaves him with making the decisions of their team. Izuku himself respects Fumikage's strength and the power of his Quirk, acknowledging him as one of the strongest combatants in Class 1-A. They later sit beside each other to watch the Sports Festival. Izuku later cheers on Fumikage and Tsuyu during their exam when they are up against Ectoplasm.
During the training camp, Izuku along with Mezo Shoji resolve to save Fumikage from his rampaging Dark Shadow, a quest in which they eventually succeeded. After Fumikage is kidnapped by Mr. Compress, Izuku chases after the villain in order to save Fumikage as well as Katsuki. After Katsuki is captured, Fumikage tries to comfort Izuku.
Izuku is on good terms with Eijiro, as in the U.S.J., when Izuku is wounded from trying to help All Might with Tomura, Eijiro tries to help him. Izuku is thankful though worried the latter will discover All Might's secret, but Ken Ishiyama intervened. Even early on in their first semester, Kirishima is sometimes shown to intervene when Bakugou starts antagonizing Izuku.
In U.A. Sports Festival Arc, when Shoto tells Izuku that he will defeat him, Eijiro tries to stop them from any further arguing, and after Izuku starts to talk negatively about himself, Eijiro tries to encourage him. Eijiro also tells Izuku that he is envious of his Quirk.
Eijiro seems to admire Izuku's growing confidence and seems to be impressed by his quick thinking and logical reasoning. He also seems capable of understanding Izuku's feelings to some extent, understanding how he feels when he is unable to rescue Katsuki. Eijiro also asks Izuku to help him in his plans to rescue Katsuki from the League of Villains. Izuku respects Eijiro by admiring his determination and he is aware of the bond Eijiro shares with Katsuki, which is what gives him the idea that Eijiro should be the one to retrieve him.
In the Shie Hassaikai Arc, Eijiro and Izuku are shown to be on even better terms than previously, often interacting in friendly ways. They also take part in the Shie Hassaikai Raid and defeating several villains.
The two first meet at the entrance exams where Yuga saves Izuku from being attacked by a robot, though he does say he only did this for points, as he remarks that the latter makes good bait and that he would most likely not see him again as he doesn’t think he’ll be accepted into U.A.. Izuku later excels in the exams and therefore becomes classmates with Yuga. It has been shown his opinion has not changed, as he again remarks on Izuku's lack of skill to Ochaco. However, Izuku surprises him with his powerful Quirk which allows him to remain in the school.
After this, the two have no further interaction that was notable until the Remedial Course Arc. Yuga acts strangely around Izuku, who would note that he can never get a good read off the latter. Izuku is later scared when Yuga spies on him as he sleeps and leaves a message (in cheese) to say he knows Izuku's secret, which left Izuku wondering if Yuga is aware of the truth about his Quirk. In training, Izuku is called out to by Yuga and is shown one of his new moves, though it causes Yuga severe stomach problems to use. Seeing him in this state, Izuku takes Yuga away to recuperate before he asks him what he meant in his message.
Yuga explains that he is aware of Izuku not being accustomed to his Quirk and reveals that he has gone through a similar experience and compares his powers leaking out in his childhood out to Izuku's initial lack of control of his own Quirk. He then tells Izuku that he is not alone in hardships, as it is better to face them together or they won't excel. His words relieve and cause Izuku to smile and thank him for his support. After a few days, Izuku and Yuga become good friends, with some of the class noting on their more frequent interactions.
During the School Festival, Izuku is tasked with securing Yuga as the disco ball since the class knew of their bond.
Izuku gets along very well with Minoru, though like most of his classmates, he is put off and disgusted by Minoru’s perversions, although he still admires his heroism and quick thinking in difficult times. In turn, Minoru has come to admire Izuku for his acts of heroism and for believing in his abilities as a hero, though he is somewhat skeptical at first of Izuku's determination. The two have become friends, particularly following their battle together against the League of Villains.
It appears he has picked up some of Izuku's habits, as he uses his own knowledge of heroes' Quirks to defeat Midnight. Witnessing this, Izuku is impressed by Minoru's ingenuity and praises him for it.
They get along well, though Mashirao didn't team up with him since Izuku was a target in U.A. Sports Festival Arc. Despite this, Mashirao still tries to warn him about Hitoshi’s Quirk, Izuku also standing up for him after Hitoshi insults him. Mashirao worries for him but is relieved when he emerges from the fight victorious.
He also tries to comfort Izuku at the training camp after he was the only one left without a partner.
The two have had little interaction but are on good terms and have respect for each other. Shoji saves Izuku twice during the school trip, first from a rampaging Dark Shadow and second from Himiko. Mezo trusts Izuku, as he is willing to lead the out of control Dark Shadow towards where Shoto and Katsuki were despite the danger it placed him in, as that was Izuku’s plan. Mezo also carries an injured Izuku on his back throughout the attack during the training camp.
Despite their efforts they fail to save Katsuki, resulting in Izuku breaking down. Along with Fumikage, Mezo tries to comfort Izuku on their loss. Later he visits Izuku in the hospital with the rest of the class.
Izuku and Denki are on good terms and they're sometimes seen talking together. Denki also considers Izuku as "very diligent". He is also seen to praise Izuku when the latter saves All Might by using for the first time, his foot style during training to create ultimate moves. Denki was shocked when he learned from Sero that Izuku was spotted behind the rocks with a naked Shiketsu girl during the first phase of the Provisional Hero Licence Exam. He then tells Izuku that he misjudged his innocence.
Despite not having too much interaction with each other, they mutually respect each other. Kyoka is amazed by Izuku overcoming his drawbacks of his Quirk.
Kyoka also finds Izuku's notebooks where Izuku notes his classmates' abilities very interesting and impressive.
Mina and Izuku have rarely shared screen time together, but when they do Mina has a playful bounce off of Izuku's muttering. Their relationship improves during the weeks leading up to the School Festival, when, interested in Mina's dance skills, Izuku asks her to teach him some dance movements, an job that Mina happily accepts.
After their team wins the fifth round in the Joint Training Battle, Mina teases both him and Ochaco for the latter grabbing onto Izuku so hard in her attempt to save him, which causes both Ochaco and Izuku to turn bright red in embarrassment.
They are classmates of Class 1-A and are both voted as the Class reps, however their partnership does not last long due to Izuku stepping down for Tenya to replace him.
There is not much interaction between the two after, though Izuku, along with Tenya and Momo, would later join Eijiro and Shoto to rescue Katsuki. Due to the group's teamwork, they are successful.
Both are also friends of Shoto and were seen showing concern for him after he failed the Provisional Hero License Exam.
He is on good terms with most of Class 1-B, except Neito Monoma who resents the class.
During lunch time, he intentionally bumped into a rambling Izuku’s head and sarcastically apologizes to him, commenting that his head was so big that he just couldn’t help but to bump into it. Izuku was silent as Neito insults him and the rest of Class 1-A, saying that they only want to gain even more attention just like they did in Sports Festival and when Izuku, Shoto, and Tenya encountered Stain, before being knocked out by Itsuka Kendo twice. Izuku only commented on Neito's behavior being strange.
They clash during the fifth round of the Joint Training Battle, Neito regards himself as a “side character” and Izuku as a “main character”. Neito manages to use One for All: Full Cowl after touching Izuku, but is pinned down by Ochaco before even getting the chance to attack him. Izuku was even worried at what might happen if his Quirk was copied.
Even though Class 1-B loses the match, Neito still retains his old demeanor. Neito begins to rant about how his team didn't lose in terms of substance and reveals his newfound knowledge relating to Izuku's Quirk. He implies that he could easily beat Izuku if they were to rematch. Despite his disrespectful and harsh attitude during the battle, Izuku does not seem to hold any grudge against Neito, in fact he was worried about him when he tried to use One For All, fearing for his physical integrity.
The next day, while trying to find more out about Eri’s Rewind to help Shota Aizawa, Neito reveals aspects of One For All that Izuku himself doesn't know. Neito says that he couldn't use Izuku's Quirk properly because, just like Eri's, it requires stored-up energy to work, and as Copy can’t take that it isn’t possible for him to use it. Izuku was relieved to hear this failed, due to the repercussions of an unprepared vessel for One For All. 
Mei and Izuku first meet during the Sports Festival. Mei takes an immediate interest in Izuku when he gets first place in the Obstacle Race and as a result, got the ten million point headband for the Cavalry Battle. She sees this as an opportunity to bring attention to her inventions by using Izuku and approaches him before the battle, asking to join his team. In return for her joining, she offers him and his other teammates support equipment to aid for their battle. Her tendency to get in his personal space creeps Izuku out, though they still get along well because of their shared admiration for heroes.
After the U.A. Tournament, the two do not meet again for a while, until Izuku wants Power Loader's help in improving and redesigning his costume. Mei recognizes him during their second encounter, but doesn’t appear to remember his name, and dismisses it in order to go back to her inventions, only to return her attention back to him when he brought up his purpose for entering the workshop. She provides Izuku some options for improvements on his costume and soon creates Izuku's Costume Gamma.
Overall, the two generally get along well as Izuku has no problem talking with Mei during their first encounter. They became acquainted over a jetpack Mei creates that is similar to a hero Izuku is familiar with, Air Jet. Izuku appears to tolerate Mei better than some of his classmates, like Tenya and Ochaco, since he shares some similarities to her as he creates ways to counter the Quirks of others and rambles, and she does the same about gadgets.
It is shown that while still having trouble training, he goes to see her for a new gadget that might help him progress with his powers. She has gotten used to him, as she now remembers his last name, in contrast to her old mistake. Izuku is still friendly to her though he is put off by her habit of getting in his personal space.
Mei's invention comes in handy as Izuku uses the gloves to defeat the villain Gentle Criminal.
They first meet in the fighting tournament of the Sports Festival. Izuku learns much about Hitoshi's Quirk from Mashirao, but accidentally plays into Hitoshi's hands, though the powers of those who previously wielded One For All free him from Hitoshi's control. Hitoshi is shocked by this and tries to control Izuku again by claiming to be jealous of Izuku's Quirk because of its combat efficiency.
Izuku ignores him, though internally understands Hitoshi's situation as he himself was considered unfavorable due to having no Quirk before One For All. Before their battle in the Sports Festival, Hitoshi and Izuku are hostile to each other, but after their battle, Hitoshi seems to have gained respect for Izuku, telling him not to lose too pitifully while Izuku respects Hitoshi's goal to enter the Department of Heroics.
Afterwards, they meet again, and while Izuku is friendly to him, Hitoshi ignores his greeting. Izuku is later shocked that Hitoshi would play a role in a training exercise in the Department of Heroics. Izuku looks forward to a rematch between them and remarked to himself how fitting it was since he had visions of his Quirk both in their last fight and future fight.
When Izuku’s Quirk goes out of control during their clash in the fifth round, Hitoshi, with Ochaco Uraraka’s help, manages to stop Izuku's rampage. Thanks to that, Izuku returns to normal and the round continues. He ends up beating Hitoshi in their rematch, but is still surprised and impressed at his vast improvement. Later, Izuku thanks him and Ochaco for saving him, something Hitoshi took in stride. Izuku was also amazed that Hitoshi will be in the Hero Course for the second year.
At first, Izuku is largely put off by Mirio due to the unusual nature of their first encounter. Furthermore, Izuku is skeptical of Mirio's status as one of the "Big Three" at U.A. due to Mirio's subpar performance in the Sports Festival the previous year. This changes after Mirio gives an excellent demonstration and explanation of his Quirk and fighting technique, after which Izuku grows to have a great amount of admiration for him, even commenting that Mirio talks just like a Pro Hero already. Izuku's respect grows upon learning that Mirio was one of the originally proposed candidates for inheriting One For All. Though sometimes he is still a bit put off by Mirio's intense amiability and enthusiasm, Izuku and Mirio get along very well and are very similar in terms of heroic ideals. Mirio puts in a good word about Izuku with Sir Nighteye, and the two work together in an internship there for a while.
He was visibly disturbed when he learned that Mirio's Quirk was taken by Overhaul, and expressed anger towards the villain for that.
After Nighteye's death, when Izuku sees how optimistic Mirio is, Izuku goes as far as to propose that Mirio could take Izuku's Quirk, indicating how Izuku believes that Mirio is truly the worthier candidate for One for All, like Nighteye had said. However, Mirio turns it down as he has faith that Eri might master her Quirk to restore his and believes in the words of Nighteye about being a hero.
Izuku is impressed by this and accepts Mirio's determination, and it seems they become closer friends afterwards. The two get along and Izuku still looks up to Mirio for maintaining a smile in spite of his situation, which is something All Might also did in the face of adversity.
At first, Kota despises Izuku and Class 1-A for always working so hard to better themselves against fighting villains. He even goes as far as to hit Izuku in the crotch for trying to be friendly towards him, as he voices his dislike of heroes. In spite of this, Izuku holds no grudge and tries to get along with Kota due to their similar circumstances of both being “Quirkless” (Izuku not originally being born with a Quirk and Kota refusing to use his Quirk), though he has no luck due to a disagreement in their ideals. Because of his past, Kota ignores anything that dealt with heroes and villains, and even Quirks altogether. After finding his secret hideout, Izuku tries talking to Kota, only for him to snap at Izuku because he's never gone through what Kota has.
However, Izuku saves Kota from Muscular (who is also the same villain who killed Kota's parents) after realizing he is in danger. When Kota asks why Izuku took it upon himself to save him, especially by how he treated him, Izuku states he had seen the latter was in danger and he had to save him. Izuku shields Kota from the Muscle Augmentation villain at all costs, even going past his limitations just to keep Kota safe.
During the battle, Kota uses his Quirk out of desperation in an attempt to help Izuku. From this, it is shown Kota has gained respect for Izuku and Kota would watch Izuku overpower and defeat Muscular. After the battle, he tells Kota that his parents died to protect him and says he risked his life for him because he wants his acknowledgment as a hero. Izuku then tells Kota to help save others just as he helped save him.
At the hospital, Izuku soon gets a letter from Kota, who apologizes for his previous treatment towards him and thanks him for saving his life. Izuku later uses the letter to show All Might and his mother that he is well on his way of being a hero after having an effect on someone.
They met again when the Pussycats visited U.A., and Izuku is happy to see him and thanks him for the letter. Shino soon points Izuku out to the new shoes Kota has, which look exactly like Izuku's shoes. Kota becomes embarrassed after Izuku pointed out the match.
Izuku and Eri never fully interact, but the short time they have with one another built a powerful bond between the two. When Izuku first encounters Eri, seeing the bandages she had as well as how fearful she was towards Overhaul, Izuku tries to protect her from him until he is forced to let her go back. After learning how Eri is being experimented on by having Quirk-erasing bullets manufactured from her body, Izuku is horrified by the act and angered at himself for letting her go back to that. Shota even deduced that Izuku is going to act recklessly to try and save her even if he tried to stop him, which Izuku didn't deny.
When they finally reunite in the battle against Overhaul, Izuku immediately follows orders to try and rescue Eri, and even pushes himself to use One for All 20% to protect Eri and defeat Overhaul. Even though he is beaten down by the villain afterwards, and Eri comes back to Overhaul declaring that Izuku cannot save her, and this is not his business, Izuku declares that because Eri was crying, he would save her no matter what.
After taking Eri from Overhaul, Izuku tells her that he will not let go of her again. When he experiences her Quirk being used, and being told how dangerous it is to be with Eri, Izuku still refuses to let Eri go, explaining that because of how her Quirk has reverted the damage he has done to himself using 100% before he could even feel pain, he states that Eri possesses a "kind, gentle Quirk", something that drives Eri to tears because she is always told that her Quirk is destructive and dangerous. This also makes Izuku go as far to use One For All: Full Cowl - 100%, something that would destroy his entire body at blinding speed, to make use of Eri's Quirk, and asks her to lend him her strength.
After the battle is over and they are in the hospital, despite Izuku being discharged, he wants to stay by Eri's side until she wakes up, but is eventually convinced by Shota to go back.
Izuku later meets her again and is able to properly introduce himself to her. He and Mirio invite her to the School Festival to have fun after noticing she is still haunted by Overhaul’s influence.
When she arrives at U.A., he is excited to see her and promises to perform for her on stage.
Izuku is intent on fulfilling his promise as he prevents Gentle Criminal from attacking the school and fights the villain by himself. Izuku later performs on stage and catches a glimpse of Eri smiling, pleased at his success.
Despite the short time they've known each other, Izuku and Melissa have become well-acquainted with each other. During their initial meeting, Izuku was flustered by Melissa by how close she was to his body when examining his scars and hero costume. Once Melissa started touring him around I-Expo, Izuku quickly bonded with her over their similar interests in heroes and support tech. Izuku even preferred Melissa call him by his hero name instead of his first or last name. The two also share a goal in wanting to save people. Melissa also shared her past of being Quirkless, as Izuku related to her since they both overcame it in their own way.
During the events of the I-Island Incident, Izuku and Melissa showed great teamwork and cooperation in stopping Wolfram and regaining control of I-Island Central Tower, in a way that mirrors how All Might and David used to work in their earlier years. Izuku comes to see her as a good friend and develops a protective side to her as he defends her from any attacks made by Wolfram or his henchmen.
Gran Torino requested Izuku to be his sidekick in an internship so that he can properly teach Izuku about the One for All Quirk. Gran Torino and Izuku were off to a bad start when they first met, with Gran Torino putting up a facade of being senile and Izuku being disappointed with having to train with someone who was no longer functional. However, when Izuku moved to leave Gran Torino promptly dropped his facade and showed Izuku what he was really capable of as Izuku was amazed by his true nature. He saw the similarities between All Might and Torino's personality shifts which helped him better understand their connection and accepted the offer to train. He started to gain respect for Gran Torino, politely calling the elderly Hero "Mr. Torino". They bonded to an extent and Torino's teaching helped Izuku gain better control of his Quirk without hurting himself. Torino also protected Izuku from a Nomu while Izuku was worried for him.
Gran Torino has shown to be impressed with Izuku, stating that All Might chose a fine successor, due to Izuku's quick thinking and potential even stating that Izuku has the potential to become a "monster" due to these traits. As they parted ways, Gran Torino asked him what his name was and Izuku replied it was Deku, with Gran Torino becoming the second person to make Izuku proud of his nickname.
Izuku was very surprised to hear that All Might had a sidekick from Gran Torino. In their first meeting, he was intimidated by Sir Nighteye. The two were mostly at odds due to Sir's cold nature and skepticism of Izuku being the next Symbol of Peace. However, they both realized they were huge fans of All Might, with both having extensive knowledge of him as well as extensive collections of merchandise. Sir hired Izuku as an intern, and Izuku has been working since then to prove his worth to him.
After learning more about All Might's relationship with Nighteye, Izuku wanted to make the two meet so they can reconcile. Though Izuku wasn't able to find the time to bring it up due to the rescue mission for Eri, he got to tell him after the rescue was complete and Nighteye was taken to the hospital.
Izuku was horrified when he learned that Nighteye was going to die, and pleaded to him to keep hanging on, and cried for him as he died but not before Nighteye gave Izuku his blessing as All Might's successor.
Despite how Nighteye praised Izuku for being able to change the future, giving him hope for once, Izuku ultimately believed that Nighteye was right about Mirio being the worthier heir for One for All and even tried to pass it onto Mirio, but was rejected. Izuku also decided to live to Nighteye's expectations as a way to honor his memory.
They meet in the U.A. School Festival Arc, where the pro hero tells Izuku to not hold back against Shoto. Izuku had heard of Endeavor’s conflict All Might and is disgusted to hear Endeavor’s view of Shoto being a tool. Izuku annoys Endeavor with his declaration of not being All Might.
They meet again when Shoto tells him and Katsuki to intern under his father. Endeavor is uninterested training others besides his son but Izuku impresses him when a villain attacks and he is ready for action.
Due to them being groomed to be the successors of their respective mentors, Izuku and Tomura mutually despise each other; Tomura despises Izuku for his admiration of All Might (someone he hates) while Izuku doesn't agree with Tomura's villainous actions. During USJ, Izuku saved All Might, which caused Tomura to take an interest in Izuku as shown when he watched Izuku's match during U.A.'s Sports Festival.
Later in Kiyashi Ward Shopping Mall, they unintentionally met each other again, so Tomura decided to talk with Izuku about the difference between him and Stain. After hearing Izuku's thoughts on the matter and before leaving the shopping mall, Tomura declared that the next time they met, he would show Izuku no mercy.
Tomura is considered a dark reflection of Izuku. While Izuku is working to become the greatest hero, Tomura is working to become the greatest villain.
Himiko first encounters Izuku when the Vanguard Action Squad attacks Classes 1-A and 1-B's field training. Himiko jumps him and briefly introduces herself before she is pushed away by Mezo Shoji. Unbeknownst to Izuku, the two meet again during the Hero License Exam where Izuku believes her to be Camie Utsushimi, a student from Shiketsu High School. This encounter only raises Himiko's interest in the boy, as she notes that their interest in each other seemed to be mutual. Izuku is surprised by her fighting abilities but finds her to be a somewhat scary person the more she interacts with him.
Being saved and carried to safety in a princess carry by her love interest, Himiko found out that the boy would save somebody even if that person was his opponent. Revealing her Quirk to him but not her real identity, she was curious about how far his good will reached. During the Second Exam of the Provisional Hero License Exam begun, Izuku acknowledged her wishing of good luck to him.
After the License Exam, Izuku wanted to talk to her again about her stealth techniques but he was disappointed when he found out she had already left.
During Eri's rescue operation, he ran across her yet again when a disguised Himiko lovingly tried to cut him with her knife. Her Quirk was deactivated by Aizawa and her identity revealed giving Izuku a shock as he was saved by his teacher. Putting two and two together, Izuku figured out that Himiko was likely the same girl that he had sparred with during the license exam.
Izuku meets the villain when the latter nearly kills Kota Izumi. Muscular express surprise to recognize Izuku and told him that he has orders to kill him while Izuku resolves to face him alone. In their fight, Muscular takes glee in tormenting Izuku who pushed his body and Quirks to his limits. Izuku was also horrified to learn from Muscular that Katsuki is also a target for the League and to learn Muscular is the person who killed Kota's parents, which installed his dislike of Quirks and the hero society. Overcoming his internal struggle, Izuku fought back harder and pushed him back, leaving Muscular surprised and impressed. Muscular took the time to fight all out and overpowered Izuku but he is saved by Kota who distracts him enough for Izuku to use One For All at 1,000,000%. With an amplified Delaware Smash, Izuku is able to break apart Muscular's muscle fibers and send him flying deep into the rock wall, finally defeating the villain. 
Muscular was one of the last villain Izuku defeated before getting his provisional license and gives him a sign on the bodily damages done onto his body if he exerts himself. This also causes him to develop a new fighting style with his kicks.
While interning with Sir Nighteye, Izuku learned of the villain and his connection with the Yakuza. Not too soon, Izuku meets him in person when Eri ran to him for help and Chisaki arrived shortly after. Izuku immediately recognized him and was nervous to find him so easily, Mirio helped him to maintain his cover and the villain seems doesn't appear to notice they are aware of his identity. Izuku wanted to question him on his treatment of Eri, Overhaul appeared to be ready to attack but Eri runs back to him.
While the heroes were doing investigations, Izuku and Mirio both agree they have to stop Overhaul and save Eri from him
Izuku went beyond his limits with One For All by using 100% and having Eri continuously rewind the damage done to his body. In the struggle, Izuku and Chisaki were both relentless but Izuku emerges as the victor. Despite his victory, Izuku was nearly killed by the spiteful villain but Eri saves him and Overhaul is left defeated. 
Overhaul is the first major villain Izuku fought after he obtained his hero provisional license, though Izuku had help from Eri to defeat him.
Stain respects Izuku's reasoning to be a Hero and sees worth in letting Izuku live. Izuku, on the other hand, saved Tenya from Stain after intervening in the man's attempt to kill his friend. Izuku impressed Stain with his will to defend his friend and efforts to defeat him, which was something he deemed worthy as a hero. With the help of Tenya and Shoto Todoroki, Izuku managed to defeat the villain.
Also, when Izuku was captured by one of the Nomu-like creatures, Stain was the one who saved him as the latter expressed shock at this act and was scared at Stain's speech before he fell unconscious. While Izuku does not approve of Stain's nihilistic methods, Izuku respects Stain for living according to his ideals and never giving up on achieving his ideal world.
Because of their encounter, Shuichi Iguchi prevented his comrade Magne from killing Izuku declaring that he was the person that Stain saw fit to save thus being a man worthy of the title of a true hero.
Izuku first saw him on the internet and was confused by his crimes even wondering about his intentions.
Izuku unintentionally met him in person after accidentally bumping into him outside of school when getting supplies. Initially, Izuku did not recognize him until he heard his voice and remembered his habits. Izuku begged Gentle not to attack the festival, as the villain saw Izuku as an obstacle that had to be removed, but after learning that he was a student of U.A high school he developed an interest in him.
As they fought more, Izuku was knocked down many times but wanted to honor his promise to Eri. His tenacity caused Gentle to gain a begrudging respect for him and steadily confined him more of his past. After he lost the match and hearing of Izuku's dream, he treated the boy as an equal and told him to keep moving forward. Izuku still treated him like a threat and remains hostile to him however, the more they fought and hearing of Gentle's past, he treated him with more respect and care even stating he wouldn't laugh at his ambitions.
After defeating Gentle, he remained worried about him, even after being blasted into the air by him.
Gentle is the first villain Izuku officially defeated on his own.