Yū Otosaka begins as a self-serving teenage boy wielding a power to possess others for five seconds, exploiting it to cheat academically and manipulate peers while masquerading as a model student. His guarded persona hides a fiercely protective side reserved solely for his younger sister, Ayumi, whose presence softens his otherwise callous indifference toward others. Ayumi’s tragic death—triggered by her own unstable ability—shatters Yū, plunging him into self-destructive isolation and reckless violence. His spiral into near-sadistic behavior, marked by provoking brutal gang fights, halts only when Nao Tomori intervenes, anchoring him back to school life and redirecting his powers toward protecting fellow ability users.

Joining the student council, Yū transitions from reluctant ally to strategic leader, leveraging sharp intellect and adaptability to shield others. Bonds with peers like Nao cultivate dormant empathy, solidifying his vow to safeguard ability users worldwide. This resolve drives him to steal every global ability—a mission that erodes his psyche. Prolonged power accumulation fractures his mind, inflicting memory loss, emotional detachment, and PTSD, morphing him into the merciless "One-Eyed Grim Reaper," defined by clinical efficiency.

Physical scars mirror his turmoil: a wounded right eye from a rescue mission necessitates an eyepatch, later obscured by unkempt hair. Completing his goal strips him of all memories, leaving a hollow shell. Yet flickers of innate kindness surface when he spares a final target and comforts Nao, despite unrecognizing her.

Yū’s core ability, "Plunder," transcends fleeting possession, enabling permanent theft of powers like telekinesis, pyrokinesis, and time manipulation. Each acquisition accelerates his mental unraveling. His dynamic with Nao evolves from antagonism to unspoken romance, hinted through subtle gestures before his amnesia.

Family crucially shapes his path: older brother Shunsuke, a time-leaper, models responsibility, while Ayumi’s death—and eventual revival via time alteration—fuels Yū’s desperate crusade. Childhood flashbacks reveal a nurturing, family-centric boy, starkly contrasting his later defiance.

In the end, Yū sacrifices his identity to fulfill his pledge, emerging devoid of memory yet clinging to an instinctive compassion—proof that beneath the Reaper’s void, traces of the brother who loved Ayumi endure.