Nana Shimura was All Might's mentor and his predecessor, the 7th inheritor of the One For All Quirk. Nana was the one that told All Might that a true hero should smile no matter what, to reassure the people and to not only protect their lives but also their hearts. All Might greatly respected Nana as a mentor, friend and hero, to the point he even considered her as his own mother. In the final moments of her battle with All For One, she instructed Gran Torino to get Toshinori to safety, despite his protests. He was absolutely crushed when he witnessed her being killed.
At their first meeting, All Might merely thought of Izuku as just a fan. He came to his aid when a villain attacked him and signed his notebook.
While he is usually worried about people seeing his true form, he was calmer in telling Izuku about it after the boy was shocked to see him in that state. Though he somewhat broke Izuku's spirits when he told him being powerless wouldn't help him be a hero, his opinion changed, and he started to acknowledge him when he jumped to save Katsuki Bakugo from the Sludge Villain.
After All Might defeated the villain, he told Izuku that he had what it took to be a hero. He then told Izuku that he was worthy to inherit his Quirk, One For All. He stated that he has no regrets passing One For All to Izuku.
Upon hearing Sir's prediction of All Might's approaching death, All Might openly accepts his fate but he states that Izuku's will and growth, and constant surpassing of his expectations gives him the desire and reason to live long to see his successor become the hero he wishes to be. All Might is consistently amazed at Izuku's growth and determination to become a hero and take All Might's place as the symbol of peace, so much so that after his final battle with All For One he begged Izuku to become the next symbol of peace while on his knees showing his utter faith in the boys potential. He has also called Izuku "Cool" when Izuku came up with a way to use One For All without breaking his whole arm on the fly.
His relationship with Izuku goes so far as to border on the familial. Much like how All Might came to see his own predecessor, Nana Shimura, as his own mother, he often treats Izuku like the son he never had, attempting to shape him into not only an admirable hero, but also an admirable person. He is even prone to showing Izuku open affection in many ways, such as openly hugging him and reminding him that their bond is much deeper than blood as bearers of One For All. This familial bond has even caused members of U.A.'s faculty to question whether or not All Might has been in Izuku's corner from the start.
All Might first met him when Katsuki was attacked by the Sludge Villain. They met again when in the training matches. During the Izuku and Katsuki's training match All Might acknowledges Katsuki as a promising hero.
However, he does not approve of his reckless and cruel fighting style, as he greatly scolded the boy for using an attack on Izuku that could have killed him. He often tries to give him advice which usually falls on deaf ears due to Katsuki's prideful nature. As the series continues, Toshinori and Katsuki's standing improves with Katsuki continuing to look up to All Might and All Might continuing to support Katsuki's development as a hero.
Following Katsuki's kidnapping by the league of villains and seeing All Might's true form, their relationship changes with Katsuki discovering the true nature of One For All and his taking Izuku under his wing.
During a fight with Izuku, Katsuki showed he blamed himself for All Might's loss of his Quirk however, All Might witnessed it and comforted Katsuki by reassuring him that such events were out of his control and apologized for putting such a burden on Katsuki. He stated he believes Katsuki can become an excellent hero. All Might then revealed the truth of his Quirk and him passing it onto Izuku, which showed he trusted him with this fact. In response, Katsuki promised to keep All Might and Izuku's secret.
During Joint Training Arc, All Might watched Katsuki's battle. and saw his new development. After the battle, he took time to compliment Katsuki's willingness to cooperate with his team, which led to his victory while Katsuki pridefully tells him to not dwell on it. At the end of the arc, All Might has come to include Katsuki in his private talks with Izuku over his Quirk. He allows Katsuki to fight with Izuku to test the recent phenomenon Izuku developed in his fight.
Before meeting Izuku, Principal Nezu suggested Mirio to be All Might's successor. While All Might respects and admires the aspiring hero and admits to Midoriya that he along with many U.A. students would be a worthy successor, All Might continues to believe he made the right choice in passing over students like Mirio in favor of Izuku.
Not much is known of their past, but due to their contrasting personalities, they do not get along very well, with Shota disliking heroes that lap up the media spotlight being a major point of contention. All Might and Shota also frequently disagree with teaching methods, with All Might criticizing Shota's harsher attitude towards their students.
Nevertheless, the two do show mutual respect for each other and their abilities, and both being willing to risk their personal safety to protect the other such as in the U.S.J. Arc. All Might was relieved to hear Shota was alright after he was treated at the hospital.
Over the course of the series, Shota and All Might's relationship appears to have gotten a little better, particularly after All Might's retirement. Shota counseled with All Might regarding their job to have students enter the dormitories. The two also offered to buy each other a drink, indicating they are both willing to socialize together casually.
Recovery Girl is one of All Might's friends, and also one of the few people who know about his condition and Quirk. She frequently scolds him for letting Izuku damage himself in fights and for not being a good enough mentor, believing that All Might spoils the boy. Following the final exams where All Might faces Izuku and Katsuki, Recovery Girl scolds All Might for his lack of restraint against the two. Despite these disagreements on All Might's teaching style, Recovery Girl still considers All Might to be both a friend and an excellent hero.
Nezu is another of the few people that knows of his condition and was the person that offered him a teaching position at U.A. The two have an amiable and mutually respectful relationship, although All Might often finds himself trapped by the principal's long winded and boring speeches and Nezu often has to deal with All Might's heroics interfering with his job as a teacher.
Gran Torino trained All Might back when he was a student. Though he respects him as his mentor, All Might is somewhat scared about talking about him to Izuku, due to past trauma from his training methods. Despite this brutal training Gran Torino does deeply care for All Might and only became a teacher in order to tutor the young inheritor. It was suggested by Izuku that Torino is the source of All Might's split personalities in his forms.
In the Vs. Hero Killer Arc, Torino contacts him as All Might is shown nervous in the talk as he gives his old mentor praise for his strength. Torino told him if he felt that way, All Might would have visited him as the latter gets more nervous. Despite that, Torino complimented All Might on picking a successor like Izuku as he saw the same flame in him that he saw.
During the Hideout Raid Arc, he fought alongside his mentor against the League of Villains. When All Might fought All for One, Gran Torino joined All Might in his final battle, acting not only as an ally in combat, but as an emotional rock, keeping All Might focused in the face of All For One's provocations and taunts. Torino also expressed his pride in his pupil for defeating All For One a second time while being the Symbol of Peace one last time. After All Might discovered Tomura Shigaraki was Nana Shimura's grandson, Torino stopped him from going after him as it would be fruitless to try in reasoning with a criminal and All Might accepted his teacher's words.
Ten years before the start of the series, All Might once had a talk with Endeavor and since then, they have not spoken to each other. All Might appears to respect Endeavor, wanting to get advice from him on how to raise the next generation. It seems that their rivalry is mainly one-sided (on Endeavor's) as All Might is usually friendly with him.
After All Might's retirement and Endeavor's subsequent promotion to No 1 hero, the two appear to be on better terms, with Endeavor approaching the former hero on how to fill the void left by All Might. Endeavor even thanked All Might for bringing his son back to him, which All Might stated wasn't a problem. Later on, he sincerely asked him what it truly is to be the Symbol of Peace.
All Might says he simply does not know how to answer. All Might states that he always believed that the country needed a symbol, a beacon of hope and a warning to villains, and in doing so he ignored the people around him, such as his former sidekick. All Might tells Endeavor that he knows his struggle of filling in his spot as the Number One Hero and how people keep comparing them, but also tells him he does not need to follow the same path that he took to become the same symbol but find his own way of doing things instead.
Sir Nighteye was once All Might's sidekick before their eventual falling out when their values conflicted in regards to All Might's injuries. Nighteye pleaded with All Might to retire and find a successor. Nighteye believed that by retiring, All Might's legend would live on and that he would only damage the citizens and other heroes who look up to him if he pushed himself. All Might refused his partner's advice for fear of another villain as powerful as All For One rising to power in his absence.
During their argument, Nighteye brought up a future he saw of All Might via his Quirk, where All Might would clash with a villain and meet a gruesome demise. As his foresight was never wrong, he did not want to see All Might's death. From there, they had a fight and separated.
Despite this, Nighteye doesn't hold any resentment against All Might, and still admires him and cares for him despite their disagreements. However, All Might was reluctant with the idea of meeting his old sidekick again due to becoming something Nighteye warned him about. This is also one of the reasons why he refuses to help Izuku to getting an introduction to Nighteye. Equally, in spite of all the admiration that he professes to him, Sir Nighteye thinks that All Might's decision to choose to Izuku as his successor was a mistake.
Very soon, Nighteye was critically injured in a battle and All Might made the choice to see his old friend one last time. He was sad as Nighteye laid in pain as he begged the latter to hang on so he can properly make amends but Nighteye he had already forgiven All Might and was happy he just came to see him. Nighteye stated he saw the potential of Izuku that All Might saw and deemed him worthy of being the latter's successor. When Nighteye died, All Might shed tears at the loss of his old friend.
Naomasa is also another person that knows about his condition; All Might thinks he is the best cop there is.
Naomasa and All Might met while the latter (in base form) was posing as All Might's contact, in recommendation of Naomasa's partner. While they were having a conversation in a cafe, All Might constantly made excuses and run to perform quick deeds as All Might. When he accidentally returns with the uniform still visible (revealing his dual identity as All Might), Naomasa calls him out on not filing reports on all of his heroic activities (as customary of a Pro Hero). Taking pity on the number 1 Hero, Naomasa offered to file the reports himself, letting All Might act without any worries apart from telling Naomasa what he was doing.
They have since remained good friends with Naomasa being one of the few individuals that All Might fully trusts and can confide with. Professionally the two also have a special relationship with Naomasa revealing information directly to All Might on matters such as All For One and Nomu.
David Shield was All Might's partner when he worked in the US. After All Might rescued Dave from a fire-related accident, the two became friends. They had a very close relationship during their college days, to where David refers to All Might as "Toshi" and even invited him to his wedding (as seen in one of Melissa´s photos). David was also the designer and creator of every single one of All Might's hero costumes. All Might seems very appreciative of the things that David has done for him, stating that "your father is my hero" to Melissa Shield.
David still appears to have strong feelings for All Might during the events of Two Heroes. He can often be seen looking at an image of a younger All Might on his phone throughout the film, and Melissa even described him as "someone you love" when bringing All Might into his lab to visit him after his arrival at I-Island. David also appears to miss All Might, stating that he wished that All Might had stayed in America. (in the English dub, he says "America needs you"). It appears that they still keep in touch via email.
David is aware that All Might is getting weaker, but he isn't aware about the secrets of One For All. It is later revealed that David was creating a Quirk-amplifying device to assist All Might, and allow him to keep being the Symbol of Peace for a longer time. He even went so far as to recruit villains to help him with his plan, unaware that All For One manipulated from behind the scenes to plummet David's reputation, all to spite All Might.
At first, All Might detested Tomura for invading U.A. and trying to kill the students. He considered him as a criminal with a strong desire to cause random acts of destruction and murder, but one with a "man-child" personality who likes to brag and acts out when things do not go his way. However, things changed when All For One told him that Tomura is the grandson of his mentor, which caused him a great shock.
All Might wishes to find Tomura and stop him; however, Gran Torino tells him that he is in no position to do such a thing and can't do that because he would not be able to see Tomura as a villain.
All Might and All For One are bitter enemies and detest each other; All Might despises All For One's antagonistic and manipulative nature while All For One despises All Might's heroism. All Might fought All For One five years ago and defeated him, which furthered All For One's hatred for All Might. All For One is an incredibly powerful villain and gave All Might his critical wounds that limit his ability to perform his duties as a hero. All Might is very cautious about All For One, going as far as to warn Izuku of the dangers that he poses. He seems to also place his hand on the wound All For One dealt him when talking about him. In their final battle All For One continually taunted All Might, knowing where to strike to enrage and wound All Might, bringing up his former master and her grandson.
During All Might's visit to see his nemesis in Tartarus Prison, All For One gave All Might insight into his reasons for adopting and apprenticing Tomura Shigaraki. He showed a penchant for manipulation, enjoying playing mind games with All Might in an attempt to provoke him into leaking information from the outside world. He also mentioned that his life-threatening injuries that left him blinded and scarred are why he worked so hard to cultivate a plan for empowering Tomura. In turn, All Might simply turned away and promised that he would still do everything he could to interfere with All For One's ambitions, showing that their enmity had only fermented and grown in potency.
All Might meets Izuku's mother for the first time when he goes to their apartment to talk to her about the dorm system. Inko is amazed that the former pro-hero decided to visit her home and is nervous by his arrival. She reveals that she's against it and wants to pull her son out of U.A. High School, because she does not trust that they can protect her son. Inko wishes to support her son in his dream, but his mounting injuries as well as the school being attacked multiple times has caused her lose confidence. Also, she knows that Izuku looks up to All Might so much that she is worried this will lead him down the same blood filled path.
Izuku is shocked by his mother's revelation, but he understands her and told her there would be no problem as long as he managed to become a hero in another school. All Might transforms into his muscular form and bows before Inko and Izuku. All Might apologizes to Inko for his negligence as Izuku's teacher and understands that she is worried about the bloody path of a Hero, but reveals that he believes Izuku should become the new symbol of peace. All Might returns to his weak form and expresses his desire to raise Izuku to prevent him from walking down a blood-stained path to heroism. He asks that Inko look beyond the current U.A. and look to a future U.A. that's trying to change for its students. Finally, All Might asks that she allow him to pour his all into Izuku, and even promises to protect and raise Izuku, even at the cost of his own life.
Inko is shocked at All Might's resolve. She states that all she wants is Izuku to be happy and tells All Might that he should never trade his life away for someone else's sake. She asks All Might to live properly and raise her son. If All Might promises her that, she will allow Izuku to attend U.A. High again. All Might promises to do so and Inko asks that he take good care of her son. Izuku promises to not make his mother worry any longer.
Outside, after Midoriya and All Might manage to convince her to let Midoriya stay at U.A, All Might compliments Inko for being a wonderful parent, telling Izuku that she reminds him of his master Nana.
All Might met Melissa when she was still a child, during the years he was working as a hero in the United States, with her father as an ally. He tells her that, despite her being Quirkless, she can be a hero to other people in other ways, just as her father is to him as he is the only one capable of producing to him the right equipment.Hearing how her father was a hero to All Might inspired her to follow her dad's footsteps and become a support engineer.
Later on, she becomes familiar with All Might enough to refer to him as "Uncle Might" when she invites him over to I-Island in email.