Yuichi Katagiri is a second-year high school student living independently, supporting himself through part-time work due to poverty. He initially lives by the principle "friends are more important than money," instilled by his adoptive mother Yuka Katagiri, driving his fierce loyalty to his close friends: Shiho Sawaragi, Tenji Mikasa, Makoto Shibe, and Yutori Kokorogi. His early life was unstable and traumatic. Raised by Yuka, who turned to sex work for income, Yuichi also endured a manipulative father figure, Taizen Shiba—a con artist who taught him psychological manipulation and mentalist techniques. Though initially admiring Taizen, Yuichi later despised him, labeling him "devil" and "scum," and ultimately killed him. Taizen's death was one of three murders Yuichi committed; the other victims remain unnamed. Afterward, Yuichi reentered mainstream society, attending school while earning a living making paper flowers. He eventually formed his high school friend group, though their bonds fractured when forced into the Tomodachi Game. Notably, Yuichi himself founded the Tomodachi Game during childhood, though its origins are unspecified. Physically, Yuichi is relatively tall and lanky with sharp features and messy black hair, evolving from a short, scruffy childhood style to longer adolescent bangs. His expressions vary dramatically, shifting from soft and happy during friendly interactions to a distinct "insane slasher smile" during serious gameplay. He typically wears his high school uniform or game-specific attire. Yuichi's personality undergoes profound shifts. Initially presenting as diligent, kind, and self-sacrificing—even accepting blame as a traitor to protect friends—he gradually reveals a darker, sinister side forged by trauma. This manifests as ruthless pragmatism, near-total absence of remorse for morally questionable acts, and occasional sadistic pleasure in enemies' suffering. He employs threats, blackmail, and psychological torture to achieve goals, like intimidating Kaidou or manipulating Kokorogi to coerce a teacher. Despite this, he maintains a protective stance towards his core friends, though his methods blur moral lines. His abilities are central to his effectiveness in the Tomodachi Game. Though academically unremarkable and self-admittedly inferior in raw cognition to rivals like Kei Shinomiya, Yuichi possesses exceptional emotional intelligence (EQ) and practical knowledge of human psychology and criminal tactics. His IQ is estimated between 145–150. He excels as a master manipulator, strategically allowing adversaries to underestimate him while crafting intricate schemes exploiting emotional vulnerabilities, aided by strong acting and deception skills that let him feign vulnerability or alignment. Physically, he shows decent athleticism in running and combat, but his most notable trait is extreme pain tolerance. He endures repeated punches, having three nails torn off, a knife through his hand without breaking composure, and deliberately breaking his own finger to corner an opponent. His ruthlessness—described by others as "evil," "monster," or "devil"—is a tactical asset; he prioritizes victory above all else against non-friends, including willingness to kill administrators if necessary. Throughout the narrative, Yuichi evolves from a seemingly ordinary student into a morally ambiguous strategist whose loyalty to friends coexists with a capacity for brutality and manipulation, continually challenging themes of trust, betrayal, and the monetary value of relationships.