Osamu Mikumo/Relationships

Kasumi Mikumo
Not much of their relationship has been seen, but Kasumi initially disapproved of her son joining Border. After noticing how nobody who visited Osamu in the hospital after the Second Large-Scale Invasion discouraged his participation in Border and watching his speech during the press conference, she changed her mind. She states that he is free to make his own decisions in life, but she also guaranteed that she would forcibly drag him away from Border should he ever grow weary of it. She also seems to believe Osamu and Chika's relationship is more than friendship, and repeatedly tries to push them together.

Rinji Amatori
Rinji was Osamu's tutor and Chika's older brother, making him a person Osamu looked up to. He promised to Rinji to look after Chika, a task that Rinji explicitly entrusted him with before going missing through a Gate. Since then, Osamu took his role as Chika's guardian seriously, even joining Border in hopes of better protecting her. While he mostly aims for the Away Mission due to Chika's predilection to do so, Osamu has also expressed concern for Rinji's wellbeing and circumstances.

Yūma Kuga
Upon meeting him as a new classmate, Osamu was the first person Yūma grew to trust on Earth due to the former's honest nature, one that was greatly apparent to Yūma's through his Side Effect. Osamu would be the first people Yūma confided about his origin, and would later be grateful that Osamu was the first Border Agent he happened to meet. Choosing to trust him about maintaining secrecy and keeping a low-profile, Osamu would act as Yūma's voice of reason, teaching him the customs of Earth and Japan, helping him acclimatize to his new environment.

Yūma was puzzled by Osamu's kindness at first, but came to accept it as a part of his character. Yūma seems to draw parallels between Osamu's reckless altruism and the final act of his late father, Yūgo. As such, he doesn't fully understand where their drive to sacrifice themselves comes from, but hopes to understand it. It was partially out of concern that his indiscriminate kindness would get him killed that Yūma agreed to join his team on Border.

With time, the two would come to better understand each other and become great friends, with Yūma risking his life to aid Osamu during Aftokrator's Invasion; first by sending Replica to help him, and later by rushing there himself. In the aftermath of the invasion and the press conference, Osamu addressed Yūma as his "partner", further demonstrating how their friendship has developed over time.

As fellow members of Tamakoma Second, Osamu and Yūma rely on each other in battle, with Osamu placing great faith in Yūma's individual power, and Yūma trusting Osamu's strategies and judgments as Captain, best shown in how he hasn't contested any of Osamu's decisions till date. Yūma doesn't hold Osamu's lack of battle prowess against him in the slightest, having no reprimand for his failings in Round 4 of the B-Rank Wars. The duo would prove to be a powerful tag team, with some of Tamakoma Second's best strategies and victories relying on their coordination (such as their defeats of Kōtarō Suwa, Katori Squad, and most notably Masataka Ninomiya).

Chika Amatori
Chika is the younger sister of Osamu's tutor, who had Osamu promise to protect her in his absence, which was part of the reason he joined Border. He was the only person whom she trusted—specifically about her Side Effects and the fact she was targeted by Neighbors—after the disappearance of her friend and brother, at least before she joined Border. He is always encouraging her, and she places much trust in him, too. When Usami was asking for Chika's strengths, Osamu was able to quickly assess her strong points, indicating he knew her quite well even before they were teammates.

As Border Agents, Chika follows Osamu's orders as Captain, and places great faith in the strategies he enacts. For his part, Osamu (as well as Yūma), was quite concerned by the Chika's lack of self-worth and propensity for offering herself up as a sacrifice, which led him to explicitly order Chika to prioritise her own safety, even during the Rank Wars, which were entirely safe environments meant for training. Tamakoma Second's entire purpose is to support Chika's goal to go on the Away Mission and search for her brother and friend, a goal that Osamu and Yūma both endorse, even though Osamu was initially concerned when she decided to join Border.

Osamu's concern for Chika's wellbeing is reciprocated, as Chika felt incredibly guilty when Osamu was injured protecting her during the Second Large-Scale Invasion, and would constantly worry if she was holding her team back due to her inability to shoot people. According to Shūji Miwa, the problem wasn't Chika's resolve, but Osamu not having the heart to give her the order out of concern for her feelings. In the end, it was the sight of Osamu in danger that allowed Chika to overcome her block during the Final Round of the B-Rank Wars.

Yūichi Jin
Osamu first met Jin when the latter saved him from a Neighbor back during Osamu's break-in of Border HQ. Jin would help Osamu join Border as a combatant, something Osamu would gratefully reflect back on many times in the future. He recognized Jin immediately when he saw him again, and expressed relief that Jin remembered him as well. The former S-Rank agent has referred to Osamu as "Four Eyes" from the day they met.

Osamu holds Jin to generally high regard, looking up to him as a Border agent and admiring both his strength in battle and his ability to get things done through connections and natural charisma. In the anime, Osamu couldn't help but compare his present self against Jin, reinforcing the impact his rescue had on him, even after all that time. Osamu's admiration of Jin would eventually lead him to request the A-Rank agent to join Tamakoma Second.

From his side, Jin is always encouraging Osamu to do his best, and looks out for his wellbeing, giving up his Black Trigger to ensure Tamakoma Second could enlist, pulling multiple strings to ensure Osamu's survival of Aftokrator's Invasion, and later laying the groundwork for Hyuse's membership in Osamu's team. Jin is one of the few people who sees potential in Osamu, and places great faith in his ability to reach his goals, seen in how he entrusted Galopoula's communication bracelet to the Shooter with the confidence that he would clear the Away Mission Test. As such, Jin is almost always in Osamu's corner, and although he couldn't take up his request to join Tamakoma Second, Jin maintains that he owes Yūma and Osamu for the aftermath of the invasion and Replica's sacrifice, intending to repay them in the future.

Kyōsuke Karasuma
Karasuma is Osamu's mentor at Tamakoma, responsible for his training and improvement. Although initially underwhelmed by Osamu's lack of combat ability, Karasuma nonetheless worked on helping him improve. Over their time as mentor and mentée, Karasuma came to grasp Osamu's character quite well, encouraging him to be more offensive in battle and later recommending him to be a Shooter instead of a Gunner after diagnosing him as "weak, but not stupid". Although not to the same level as Jin, Karasuma also believes in Osamu's potential, seeing worth where few others could in his matches against Sōya Kazama, and taking pride when his apprentice managed to force a draw against the no. 2 Attacker.

During the Second Large-Scale Invasion, Karasuma's concern over Osamu's safety was such that he tried to go against Jin's prediction and be the one to defeat Hyrein, thus averting the danger to Osamu. When this failed in spite of the All-Rounder's best efforts, the generally stoic Karasuma expressed uncharacteristic frustration.

Over the course of the B-Rank Wars, Karasuma would continue to guide and help Osamu as best as he could, such as by connecting him to Arashiyama to learn advanced Shooter techniques, helping him develop strategies by studying the logs, and later by emulating Masataka Ninomiya's trion levels and combat style in the virtual combat zone to help him prepare for Tamakoma Second's upcoming match.

Jun Arashiyama
Arashiyama became acquainted with Osamu after he saved the students of his school, which included his siblings Saho and Fuku. He praised Osamu for his good work rather than scolding him, in spite of Osamu breaking the rules of Border in that context. Arashiyama has stated that he owes Osamu a great deal when he came to support the Tamakoma Branch during the Black Trigger Capture Arc. He was also enthusiastic to teach him Shooter techniques in preparation for Round 4 of the B-Rank Wars.

Following Tamakoma Second's crushing defeat, Arashiyama was concerned that the match would have been a "bitter pill" for Osamu to swallow, recognizing its impact on the Shooter's psyche and self-esteem. Osamu would later agonize over his defeat on account of feeling guilt for having wasted the efforts of those that helped him, including Arashiyama.

Ai Kitora
Osamu respects Kitora for her superior skill and talent, and although the A-Rank ace originally chided him for breaking Border's rules and thought he was trying to draw attention to himself, she later saw how sincere he was, and even attempted to defend his actions in her report to Border's Upper Echelons. She is constantly trying to look better than Osamu, especially in front of their mentor, Karasuma. She also demands respect and recognition from him, but is surprised when he actually gives it.

In spite of her dismissal of his efforts and harsh criticism, Kitora is genuinely interested in Osamu's growth, and only scrutinizes his efforts from a position of understanding, as she doesn't want him to get invested in unrealistic goals. While she can relate to his struggles with low trion levels due to having once had the same problem, Kitora was hesitant to help Osamu as she believed inordinate kindness would only become a burden to someone with his sense of responsibility and gratitude, showing deep-rooted concern and a deeper understanding of Osamu's character, as her sentiment rang true given Osamu's guilt over his defeat in Round 4.

Upon further request, she suggested the Spider trigger to Osamu, along with tips for how to incorporate it into his strategies, which he would be extremely grateful for. After this, Kitora would take some shred of pride in his achievements, but also stated it was natural given Tamakoma Second's capabilities, showing her belief in the merit of Osamu's team, a sentiment that she held even during her earlier criticism.

Kōhei Izumi
Osamu approached Izumi on Arashiyama's referral, and requested to learn how to score on his own, and how to create composite bullets. Izumi seemed to appreciate the younger Shooter's ambition, and took up this request contingent on Osamu defeating Takeru Yuiga in one-on-one matches, and was even willing to cut the quota in half and teach Osamu while he was only partway to this goal, although the latter turned it down upon realizing where he stood. In spite of this, Izumi would continue to track Osamu's progress and follow up with him. As Izumi was the commentator for Round 5 of the B-Rank Wars, he openly hoped that Osamu would show him "something interesting" during the match.

Takeru Yuiga
Yuiga's personality can be seen as a polar opposite of Osamu's: entitled, self-important, and fundamentally cowardly. Despite this, they got along rather well when Osamu trained with him one-on-one. While this might be due to Yuiga's delight over finding someone weaker than him, it might also be due to Osamu's polite nature, which led to him to treat Yuiga with more respect than the Gunner had come to expect from his own teammates. According to Izumi, Yuiga expressed disappointment that Osamu wouldn't be sparring with him anymore. When choosing preferable teammates for the Away Mission Test, Osamu listed Yuiga as one of his options.