Osamu Dazai, a enigmatic member of the Armed Detective Agency, carries the indelible legacy of his past as the Port Mafia’s youngest executive. Recruited at 14 by Ōgai Mori following a suicide attempt, he observed Mori assassinate the previous mafia boss, cementing their bond of manipulation and mutual exploitation. His mafia career forged a reputation for ruthless efficiency, tactical genius, and the merciless mentorship of Ryūnosuke Akutagawa, whose limits he tested through psychological and physical torment. Partnered with Chūya Nakahara, their destructive synergy as the "Twin Dark" crushed rival factions, though their alliance simmered with friction. The death of Sakunosuke Oda—Dazai’s sole genuine friend—triggered his defection to the Agency, spurred by Oda’s dying wish for him to protect rather than destroy. Though ostensibly reformed, Dazai’s methods remain ethically murky, blending loyalty to the Agency’s ideals with calculated manipulation of friend and foe. His nihilism persists, framing existence as inherently futile and death as an alluring release, yet he clings to a veneer of purpose through strategic gambits. Possessing the ability *No Longer Human*, which nullifies supernatural powers on contact, Dazai proves indispensable in supernatural confrontations. His intellect rivals adversaries like Fyodor Dostoevsky, matching his amorality with intricate, far-reaching schemes. Behind a mask of lethargy and theatrical suicide bids lies a meticulous strategist who orchestrates events while withholding critical intel to steer allies’ actions. Key confrontations include his imprisonment with the Hunting Dogs, where he covertly collaborated with Fyodor within Meursault to manipulate external outcomes. A staged death involving Chūya and Sigma exposed his willingness to endure severe physical and psychological tolls to dismantle Fyodor’s machinations. His mentorship of Atsushi Nakajima, emphasizing growth over fear, contrasts starkly with his earlier treatment of Akutagawa, signaling a shift in his approach to human potential. Alternate timelines, such as the *BEAST* narrative, depict Dazai entrenched as a mafia executive, underscoring the precariousness of his moral choices. Backstory explored in *Osamu Dazai and the Dark Era* delves into his mafia years, Oda’s pivotal influence, and the trauma of losing his emotional anchor. Fleeting displays of guilt and protectiveness toward colleagues betray undercurrents of unresolved turmoil beneath his detached facade. Persistent motifs include his quest for meaning, vacillating between self-annihilation and a cold resolve to safeguard Yokohama. Bandages shrouding old suicide scars mirror his psychological wounds. Though revered for near-infallible strategies, occasional misjudgments—like underestimating opponents—expose vulnerabilities, anchoring his mythic persona in flawed humanity.