Osamu Mikumo is a teenage boy of average height, distinguished by his medium-length black hair and signature glasses. While attending school, he wears a standard Japanese school uniform consisting of a blue blazer and gray slacks. Outside of school or combat, his casual attire typically includes a sweater and a striped scarf.

When using his Trion Body as a C-Rank Border agent, Osamu dons an all-white combat suit featuring Border emblems on both shoulders and a prominent yellow stripe running down the sleeves. Upon advancing to B-Rank and joining Tamakoma Second, his uniform changes to a more customized design. His B-Rank attire includes a blue jacket with a protective shoulder guard on the left side, displaying the Tamakoma Branch emblem. The left sleeve features a thick black stripe with a thinner white stripe running through its center, which connects to the shoulder guard. His squad’s emblem is displayed prominently on the left side of his chest.

Osamu Mikumo is known for his honesty, a trait noted by Yūma Kuga, whose ability allows him to detect lies. He possesses a strong sense of justice and unwavering determination, often risking his own safety to protect others. While observers such as Jin suggest that Osamu joined Border out of a desire to help people, Osamu himself views his motivations with skepticism. He believes that compromising on his principles would lead him to avoid difficult situations altogether. As a result, he views his choices not as acts of altruism, but as self-serving efforts to stay true to his ideals.

Despite lacking natural talent in combat, Osamu is deeply self-aware of his limitations and works hard to overcome them. His strategic mind and adaptability have earned him a reputation among some Border members. Kei Tachikawa describes him as ruthless in battle due to his opportunistic and unconventional tactics, while Kōji Oki observes that Osamu maintains a calm and composed attitude even in high-pressure situations such as during the climactic moments of the Second Large-Scale Invasion.

A turning point in Osamu’s development occurs after receiving advice from Sōya Kazama, who urges him to focus on his role as a team captain. Inspired by this, Osamu begins prioritizing team strength over individual performance. This shift in perspective leads him to pursue bold strategies, such as inviting Jin to join Tamakoma Second, forming alliances with former enemies like Yūma Kuga, and employing negotiation techniques he observed from Eizō Netsuki during a tense press conference. These changes highlight Osamu’s growth as a leader capable of thinking beyond conventional methods.

Trion Capacity
Osamu possesses a relatively low level of trion, the energy source used by Border agents to activate and sustain their Trion Bodies. According to Yūma Kuga, Osamu’s trion levels are so low that he would need three times his current amount to draw the attention of Neighbors. Both Replica and Kyōsuke Karasuma have remarked on the weakness of his trion output, with Karasuma initially expressing surprise that Osamu had attained B-Rank status, an achievement largely attributed to Yūma’s support and assistance during ranking battles.

Tactical Intelligence
What Osamu lacks in raw combat power, he makes up for in tactical prowess. His keen strategic mind has earned praise from high-ranking Border agents, including Sōya Kazama. Osamu is capable of crafting complex battle strategies and has outmaneuvered seasoned opponents, such as Hyrein, successfully thwarting his attempt to abduct Chika Amatori during the Aftokrator invasion. His analytical thinking also extends beyond the battlefield; he has demonstrated advanced negotiation tactics, such as controlling conversations by planting subtle cues and “agents” a method he learned from Eizō Netsuki during a press conference manipulation. These skills highlight his adaptability and growing capability as both a leader and tactician.

Rinji Amatori
Rinji Amatori, Chika’s older brother, once served as Osamu’s tutor and became a significant role model in his life. Before disappearing through a Gate, Rinji entrusted Osamu with the responsibility of looking after Chika. Honoring that promise, Osamu has since taken his role as her protector seriously going so far as to join Border with the primary goal of keeping her safe. His motivation to participate in the Away Mission is largely influenced by Chika’s own desire to go, but Osamu also continues to express concern for Rinji’s safety and circumstances.

Yūma Kuga
Osamu was the first person Yūma Kuga came to trust after arriving on Earth, a bond formed largely due to Osamu’s honesty something easily detected through Yūma’s Side Effect. Osamu was also the first person Yūma confided in about his true origins. From the beginning, Osamu acted as a guide, helping Yūma understand Earth’s customs and adjust to life in Japan. Although Yūma initially struggled to comprehend Osamu’s selfless behavior, he began to see similarities between Osamu and his late father, Yūgo, particularly their willingness to sacrifice themselves for others. Concerned that Osamu’s altruism might one day lead to his demise, Yūma ultimately decided to join Tamakoma Second to support him.

Kōhei Izumi
Seeking to improve his skills as a Shooter, Osamu approached Kōhei Izumi, a respected member of Border’s Arashiyama Squad, after being referred by Jun Arashiyama. He asked Izumi to teach him how to score independently and use composite bullets. Impressed by Osamu’s determination, Izumi agreed on the condition that Osamu first defeat Takeru Yuiga in a series of one-on-one matches. Though Izumi offered to reduce the requirement, Osamu declined, choosing instead to face the challenge head-on. Despite the outcome, Izumi continued to monitor Osamu’s progress and expressed hope during Round 5 of the B-Rank Wars that Osamu would showcase something noteworthy.

Takeru Yuiga
Takeru Yuiga’s personality stands in stark contrast to Osamu’s brash, self-centered, and often cowardly. Nevertheless, the two formed a somewhat amicable rapport during Osamu’s training sessions. While Yuiga may have enjoyed being stronger than someone for once, Osamu’s respectful demeanor likely contributed to their unlikely bond. According to Izumi, Yuiga was disappointed when their sparring sessions ended. Interestingly, during the selection process for the Away Mission Test, Osamu listed Yuiga as one of his preferred teammates, suggesting a level of mutual respect between them.

The name Osamu (修) is commonly associated with meanings such as “discipline,” “study,” or “to master,” which reflects Osamu Mikumo’s diligent and earnest personality, as well as his constant desire to improve despite his limitations.

His surname Mikumo (三雲) is written with the kanji 三 (mi), meaning “three,” and 雲 (kumo), meaning “cloud.” While the name does not have a direct narrative connection, the imagery of “three clouds” may evoke a sense of calm, movement, or subtle strength paralleling Osamu’s quiet resilience and strategic approach.