Their friendship is built upon a mutual respect towards each other. They teamed up along with Ochaco and Mei Hatsume during the Human 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 tournament. Izuku later cheered on Fumikage and Tsuyu during their Exam against Ectoplasm.
During the training camp, Izuku along with Mezo Shoji resolved to save Fumikage from his rampaging Dark Shadow in which they eventually succeeded. After Fumikage is kidnapped by Mr. Compress, Izuku chased after the villain in order to save Fumikage as well as Bakugo. After Bakugo was captured, Fumikage tried to comfort Izuku.
Although their interactions in the classroom have been minimal, Fumikage has been with Tsuyu a number of times. They worked together during the battle trial and the practical section of their final exams.
Fumikage and Tsuyu agreed to strategize prior to their practical exam, showing they had strong communication with one another. During their test against Ectoplasm, Fumikage and Tsuyu displayed elite synergy and teamwork. Their Quirks work very well together in both evasive and combat situations, and Tsuyu can compensate for Fumikage's weaknesses. Although they were overwhelmed by Ectoplasm, they managed to pass their finals.
After Mezo lost one of his arms due to Moonfish's ambush, Fumikage lost control over Dark Shadow and harmed Mezo in the process. Izuku along with Mezo resolved to save Fumikage from his rampaging Dark Shadow in which they eventually succeeded with the help of Katsuki and Shoto. Fumikage felt guilty for harming Mezo and apologized; Mezo shows no ill will towards Fumikage, thus he and Mezo are on good terms with each other. After Fumikage is kidnapped by Mr. Compress, Mezo chased after the villain in order to save Fumikage. Mezo manages to grab Fumikage's marble form after Mr. Compress spits him out due to Yuga's Navel Laser, thus Mezo saved Fumikage from the League of Villains' clutches.
It appears Fumikage and Katsuki are on neutral terms with each other as they neither hate or are friendly with one another. They first interacted during the semi finals of the Sports Festival tournament where they faced each other. Katsuki appeared to enjoy the fight with Fumikage despite Dark Shadow being annoying, although Fumikage isn't too fond of Katsuki's savage fighting style, but doesn't appear to hate him for it. Despite what he thinks of Katsuki's aggressiveness, he was nonetheless thankful towards Katsuki for helping him regain control over Dark Shadow during the training camp. Katsuki also expressed some disappointment that their Quirks had bad chemistry, possibly indicating that Katsuki views him as a worthy opponent.
They fought the League of Villains together during the U.S.J. Arc, so Koji is one of the people who knows Fumikage's weakness, in fact Koji was the first in Class 1-A to learn Dark Shadow's weakness.
They also showed good cooperative skills with each other during the Provisional Hero License Exam.
Fumikage and Shoto haven't properly interacted with each other. However, during the training camp when he lost control of Dark Shadow, Shoto and Katsuki used their Quirks to help Fumikage regain control over Dark Shadow. Fumikage thanked Shoto for saving him. Shoto was amazed with the strength of Fumikage's Dark Shadow against Moonfish. After Fumikage was kidnapped by Compress, Izuku, Shoto and Mezo with the help of Ochaco and Tsuyu chased after Compress in order to rescue him and Katsuki in which they succeeded in saving Fumikage.
Fumikage and Shihai interact for the first time during the second round of the Joint Training Battle. Both discuss the similarities of their Quirks, their mutual understanding of the darkness, and their tendency of using dark concepts and penchant for dramatic speech. This leads Shihai to declare rivalry on Fumikage, to which he corresponds.
However, Shihai's rivalry proclamation was just a psychological tactic to mislead Fumikage about his true intentions, which was to capture Yuga Aoyama. Although Fumikage could not rescue Yuga, he could later defeat Shihai in battle.
Although he nearly defeats her, Kinoko outsmarts him and defeats him with her Quirk, allowing her team to win. However, after the match she shows a kind side to him when she apologized to Fumikage for using too much of her Quirk on him and telling him to take necessary medicine to better himself, as he told her not to pity him.
Fumikage has respect for Hawks, the Pro Hero with whom he did his Hero Agency Internships and Hero Work-Studies. At first, Fumikage could barely keep up with Hawks, who essentially resolved everything by himself. Their sidekicks told him that their role is essentially to perform the clean up work after a job. Being relegated to this role caused Fumikage to ask why Hawk chose him.
When Fumikage asks Hawks about this, he replies that 20% of the reason is because both are birds, and 50% because he wanted to talk to someone from Class 1-A about the League of Villains. While he filled Hawks in on the U.S.J. Incident, frustration boiled up inside him, and he left the workplace training without having learned anything.
Fumikage then recounts his Internship with Hawks, who had invited him again. Hawks notices the improvement in Fumikage's techniques. After a patrol, Hawks takes Fumikage on a flight, and tells him that the remaining 30% of why he invited him was because he thought that Fumikage was wasting a lot of his potential. Hawks tells him that he should be able to move much more freely as a bird with wing, and that he need not be confined to the ground. With the help of Hawks, Fumikage devises a new move named, "Black Fallen Angel".
Having been trained by him, Fumikage believe that the responsibility of representing the No. 2 hero falls onto him.
Fumikage cares greatly for his master. After learning he's present at the mansion, Fumikage is shown to be worrying about Hawks. Later, after spotting Hawks' burning feathers, Fumikage races back to the mansion and saves him from Dabi, going directly against the orders of the Pro Heroes. Despite hearing from Dabi that he killed Twice, Fumikage doesn't think any less of him for it, believing he did the right thing regardless, and tells his master not to die on him with tears pouring out his eyes.