Eijiro and Katsuki are on very good terms with each other. They're often times seen together during class, and despite Katsuki's aggressive behavior, it's been indicated that the two of them are friends. Eijiro has described Katsuki as being "manly," a quality that Eijiro is known to admire.
Before getting to know each other, it is suggested that Eijiro did not admire Katsuki, as is seen in the Battle Trial Arc. Eijiro openly criticized Katsuki’s fighting tactics as “unmanly,” a discredit to Katsuki’s hit-and-run tactic on Izuku. Later on, however, while working together in the U.S.J. arc, the two team up, soon sparking Eijiro's admiration for Katsuki, and what would turn into a friendship as time goes on. Ejiro goes on to describe Katsuki as “manly” on a few occasions, a clear sign of respect for him.
During the Sports Festival, Katsuki called Eijiro "hedgehog hair", which irritated Eijiro who pointed out his similar hairstyle. Eijiro asked Katsuki to let him join him during the cavalry battle after Eijiro couldn't join Shoto Todoroki's team. Regardless, Katsuki accepted Eijiro's proposal after Eijiro mentioned taking down Izuku Midoriya's team. He later called Eijiro by his last name, showing he has come to respect or acknowledge him. Although Katsuki defeats Eijiro in battle, this only furthered his respect for Katsuki.
Due to his upbeat personality, Eijiro is usually unfazed by Katsuki’s abrasive behavior. When Katsuki’s divisive attitude arises, Eijiro often makes an effort to subdue it. Eijiro has also been known on occasion to criticize Katsuki's demeanor, as in the Provisional License Exam. During the Forest Training Camp Arc, Eijiro was very worried for Katsuki when it’s announced that the villains were looking specifically for Katsuki. He also initiates the mission to rescue Katsuki after he’s kidnapped, while calling him his friend.
As noted by Izuku, Eijiro is the only one in their rescue party who can call out to help Katsuki without hurting Katsuki's pride, which was proven true and shows that Katsuki is not intimidated by Eijiro.
During the Provisional Hero License Exam Arc, Denki stated that Katsuki had reduced the power of his explosions to avoid harming Eijiro and himself.
When Eijiro was sad about almost getting him and his classmates expelled, Katsuki paid him back for the 50,000 yen night vision goggles Eijiro bought to rescue Katsuki. When Eijiro openly voices his self-doubts, Katsuki reminded Eijiro that Eijiro proclaimed he's an unwavering horse during the Sports Festival and says Eijiro can be as strong as All Might if he refuses to go down. Eijiro later thinks of this the first time he uses Red Riot Unbreakable.
During the Joint Training Arc, Eijiro watches Katsuki cooperate with his team and lead them efficiently. He proudly tells a shocked Neito Monoma that Katsuki has developed in terms of character and is pleased by his victory.
Eijiro is likely the closest person to Katsuki and one of the only people that can be regarded as a friend, as Katsuki insults him the least, and only in a way that’s not meant to be mean. He also tolerates Eijiro‘s presence most, and doesn‘t assault him when being restrained by him.
Since they happened to be seated behind one another in class, they became friends and often make comments to each other. They were later assigned to work together along with Katsuki for the Provisional License Exam. They have similar, congenial personalities, although Eijiro is more boisterous compared to Denki, who is more mischievous.
Proof of this friendship is that Denki is the first one to congratulate Kirishima for his hero debut defeating a villain alone while he was patrolling on his first day of Hero Work-Studies at the Fat Gum Agency, showing everyone the news of his feat.
In U.A. Sports Festival Arc, after Shoto told Izuku that he will defeat him in the festival, Eijiro tried to stop any further arguing and when Izuku started to talk negatively about himself, Eijiro tried to encourage him. He also told Izuku that he is envious of his Quirk.
He also 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 felt when he was unable to rescue Katsuki.
During the Hideout Raid Arc, Eijiro asks Izuku to help him in his plans to rescue Katsuki from the League of Villains, since he knew they were close. In return, Izuku was aware of the bond Eijiro shares with Katsuki, which gave him an idea for Eijiro to be the one to retrieve him and it worked.
In the Shie Hassaikai Arc, Eijiro and Izuku are shown to be on even better terms than previously, often interacting in friendly ways.
Eijiro and Sero are good friends with each other. Hanta, Denki, and Eijiro are often seen hanging out together. The three of them will typically joke around a lot and seem to have a great sense of humor, being some more extroverted members of the class.
The friendship between the two is so strong, that after learning that Eijiro was one of his classmates who had participated in the Shie Hassaikai raid, Sero was one of those who presented more concern for his buddy, a little annoyed that he had not told nothing about that situation.
Eijiro gets along well with Mina despite not interacting with her as much as he does with some of his other classmates. The two collaborated together on Team Bakugo during the Sports Festival Arc and seemed to work well together.
It was revealed that the two went to the same middle school, during their senior year, Eijiro tried to stop two students from bullying an underclassman but did not succeed. Afterwards, Eijiro witnessed Mina step in and solve the issue by getting the student and the two bullies to break dance, diffusing the situation and ending the issue. Eijiro seems to admire Mina but also begins to feel self-conscious of the fact that Mina is exciting and heroic. This is reinforced later when he attempts to stop a villain but his legs freeze up and Mina stops the villain instead.
Kirishima later apologizes to Ashido and her classmates for not being able to do anything to stop the villain. Eventually, he and Mina both take the U.A. entrance exam together and pass. On their first day of high school, Kirishima undergoes a complete appearance change, dying his hair red and spiking it up in a style that is inspired by Crimson Riot. His hairstyle also appears to have two little horns, Mina picks up on this and decides to call themselves horn buddies.
It's at this moment that Eijiro vows to say goodbye to his pathetic old-self to her and swears that he's gonna be the kind of hero that never has any regrets. The past incident took a toll on Kirishima's self-confidence, the guilt was eating him alive. Mina realizes this so she teases him and lightens the mood by poking him repeatedly and telling him not to overload himself with pressure. She shows concern for him in her own playful way. Mina then smiles at Kirishima and tells him to tell her when he really does overcome his regrets from the past. Kirishima smiles back at her in agreement and apologizes.
Recently, Mina unveiled a new defensive Super Move called Acidman. She excitedly tells Kirishima that she created it by imitating his “Unbreakable.” Kirishima seems to be pretty flattered that he inspired the move.
Eijiro has good relations with Shoto. Previously, they barely interacted and the only significant event was when Eijiro tried to stop Shoto from declaring war on Izuku for the Sports Festival.
They interact more with the kidnapping of Katsuki, as both agreed to go rescue him even though such action would go against the rules. During the rescue mission, they worked together without any problem.
After this, it is seen that their relationship between them improves and they interact more often, supporting each other. For example, after Neito Monoma made fun how two students in 1-A failed the Provisional Hero License Exam, whereas everyone in Class 1-B passed, Shoto apologizes to his classmates for being one of those who failed, but Eijiro tries to make light to comfort him, saying that Monoma is turning it into a fake competition.
They were paired up together in the final exams against Ken Ishiyama and they got along.
When the students moved into dorms, Eijiro was jealous at the popularity Rikido was getting for baking a cake and enjoyed the cake.
Eijiro and Tetsutetsu initially had a rocky relationship because of their similar Quirks. During the U.A. Sports Festival, they met when they both survived being crushed by a 0-Point Villain. As the festival progressed, Eijiro resented being constantly compared to Tetsutetsu, like when Present Mic gave them similar intros prior to their match against one another.
Since their Quirks are very similar, the two weren't able to defeat each other during the U.A. Sports Festival, meaning that they had to compete in an arm wrestling match to decide the winner of their one-on-one. Eijiro wins and congratulates Tetsutetsu on a battle well fought, starting their eventual friendship. In Eijiro's next match, Tetsutetsu cheered for him to knock Katsuki out.
They both proved to be on good terms with each other when they ended up interning together at Fourth Kind's Hero Agency. They were initially surprised to see each other, but instead of lamenting about comparisons, they friendly greeted each other.
After the Joint Training Battle, where Eijiro played a very poor role while Tetsu was outstanding, Tetsutetsu criticizes Eijiro for presenting a defeatist attitude in front of him when comparing their fights during their respective battles, especially for comparing their respective Quirk. Although in a somewhat rough way, Tetsutetsu cheers up Eijiro, telling him that Hardening and Steel are different Quirks with their own strengths and weaknesses, and reminds Eijiro that his Quirk becomes tougher and tougher when he gets beat, without having a limit as to its hardness. These words make Eijiro recover his mood.
Eijiro and Tamaki have a senpai-kouhai relationship since they both were in an internship under Fat Gum's care. Eijiro holds a lot of respect for Tamaki as he is seen praising the latter's abilities multiple times and refusing to let go of the criminal who had previously shot Tamaki. He is also shown to be thankful towards Tamaki because it is thanks to him that Eijiro could join Fat Gum's team.
Later, in the operation to save Eri, Eijiro expresses confidence in Tamaki's abilities to take care of three enemies. In return, Tamaki respects Eijiro and compares him to Mirio saying they both have an "optimist personality."
In his middle school days, when Eijiro was questioning his dream of becoming a hero, it is an old footage of Crimson Riot that reignites his passion for heroism. Eijiro shares many of his values with Crimson Riot, most notably Crimson Riot's high regard for manliness and chivalry.
Eijiro idolizes Fourth Kind and worked under him. He greatly respects Fourth Kind and saw being punished by him as a great honor.
Eijiro faced Rappa along with Fat Gum during the Shie Hassaikai Raid. When the enormous force of Rappa's fists managed to destroy his Unbreakable, Eijiro was about to be dominated by his insecurities but when he saw Fat Gum in danger, he managed to overcome and used himself as a shield to protect Fat Gum, that allowed him to defeat Rappa.
That act of courage made Rappa respect Eijiro, to the point of helping him heal himself with the purpose that they could face each other again. That confrontation also made Eijiro respect Rappa, gaining even more motivation to train his Quirk and make him even more resistant.