Abandoned as a child, Atsushi Nakajima endured severe physical and psychological abuse at his orphanage. The director inflicted prolonged confinement with minimal food, forced self-harm, and relentless verbal degradation, emphasizing Atsushi's perceived worthlessness. This abuse forged deep-seated trauma manifesting in nightmares, flashbacks, and a critical internal monologue persisting into adulthood. Expelled at 18 under the false pretext of funding shortages, the true reason was his uncontrolled transformations into a destructive white tiger during moonlit nights, damaging orphanage property. Unaware of this ability, he believed a spectral tiger haunted him. Homeless and starving in Yokohama, near despair, his life shifted when he rescued Osamu Dazai from drowning. Dazai revealed his supernatural ability, "Beast Beneath the Moonlight," enabling transformation into a white tiger with enhanced strength, speed, durability, and regeneration. Initially triggered involuntarily under moonlight, these transformations caused destructive rampages. Dazai recruited him into the Armed Detective Agency. Welcomed into this new belonging, members gifted him clothing: a white shirt, black tie, fingerless gloves, suspenders, pants, and a belt. Early days at the agency were marked by extreme self-doubt and guilt, compounded by learning of a substantial bounty on his head. He frequently prioritized others' judgments over his own instincts, exemplified by temporarily leaving the agency to protect it. His low self-worth sometimes paralyzed him in danger, requiring allies' support to act decisively. Despite this, a strong protective instinct emerged: he used his body to shield others during a simulated bomb threat in his entrance exam and showed empathy toward enemies like Kyoka Izumi, rescuing her from forced assassination work. Missions against adversaries like the Port Mafia, the Guild, and the Hunting Dogs gradually built his confidence and combat proficiency. Key developments included gaining partial control over his tiger transformations under Yukichi Fukuzawa's ability "All Men Are Equal," allowing targeted limb shifts during battle. His partnership with Ryunosuke Akutagawa, initially adversarial, became pivotal; their combined abilities defeated Guild leader Fitzgerald aboard the Moby Dick. This alliance, strained by differing moral views, forced Atsushi to confront rigid perceptions of good and evil. Later conflicts, including clashes with the Decay of the Angel, tested his resilience. He faced Kamui alone after Akutagawa's sacrifice, experiencing profound isolation before regrouping with allies. Throughout these ordeals, his regenerative abilities proved vital, healing severe injuries like dismemberment. The fusion of his tiger form with Akutagawa's "Rashōmon" created enhanced combat capabilities, featuring elongated claws and a coat-like manifestation. His character arc centered on reconciling his traumatic past with his present purpose. He struggled with fleeting satisfaction upon learning of his abuser's death but acknowledged how adversity shaped his resolve. Despite ongoing battles with fear and self-criticism, his commitment to protecting others solidified, driven by a core belief that life's value lies in safeguarding the vulnerable.