Osamu Dazai is a tall, slender young man with dark brown wavy hair and narrow brown eyes. He typically wears a tan trench coat over a black vest and light blue striped shirt, accessorized with a bolo tie featuring a turquoise pendant, beige pants, and brown shoes. Bandages wrap most of his body except his face, hands, and feet, stemming from past suicide attempts and injuries. Recruited into the Port Mafia by Ōgai Mori at fourteen, Dazai witnessed Mori assassinate the previous boss. He rose to become the organization's youngest executive, significantly boosting its profits and earning a fearsome reputation as a strategist and torturer. During this period, he mentored Ryūnosuke Akutagawa through harsh methods and formed the highly efficient "Double Black" partnership with Chūya Nakahara. Constant exposure to violence and death fueled his search for meaning, leaving him emotionally detached and entrenched in loneliness. A turning point came when his friend Sakunosuke Oda died during a conflict orchestrated by Mori. Oda's final words urged Dazai to "save the weak" and seek a better path. This prompted Dazai to defect from the Mafia. He abandoned a mission abruptly and spent two years underground, living unemployed and drunk, before joining the Armed Detective Agency through a recommendation from Taneda of the Ministry of Internal Affairs. As an Agency detective, Dazai maintains an enigmatic and theatrical demeanor. He frequently attempts comical suicides, often proposing double suicides to women, though these acts conceal a persistent void and disillusionment with life. While appearing lazy and irritating to colleagues like Doppo Kunikida, he demonstrates exceptional strategic acumen, deductive skills, and confidence during crises. He is adept at negotiation, lock-picking, and physical combat, but notably reckless as a driver. Dazai's ability, *No Longer Human*, cancels other supernatural powers on skin contact, rendering him immune to abilities and disabling them in others. This power is always active, requiring only direct touch. His development centers on mentoring Atsushi Nakajima, whom he recruits after revealing Atsushi's tiger transformation ability. Seeing potential in Atsushi, Dazai guides him toward self-worth, paralleling Oda's influence. He orchestrates collaborations between Atsushi and Akutagawa, aiming to forge a new "Double Black" to protect Yokohama. Though claiming indifference to morality, his actions increasingly align with Oda's wish for him to "save people," employing morally ambiguous methods when necessary, such as endangering allies like Ango Sakaguchi to achieve broader goals. Later arcs reveal his ongoing rivalry with Fyodor Dostoevsky, culminating in Dazai's imprisonment in Meursault. There, he feigns death via Chūya's manipulated gunshot as part of a plan to thwart Fyodor and the Decay of the Angel, highlighting his willingness to endure extreme sacrifices for long-term strategies. Light novels and spin-offs expand his backstory: *Osamu Dazai's Entrance Exam* details his early Agency days with Kunikida; *Osamu Dazai and the Dark Era* explores his bond with Oda; *Dazai, Chūya, Fifteen Years Old* depicts his turbulent Mafia partnership with Chūya; and *BEAST* presents an alternate timeline where he remains a Mafia executive with Atsushi and Kyōka as subordinates. Accounts suggest he is haunted by memories and experiences insomnia, evidenced by alcohol use and moments of dissociation. He exhibits a compulsive need to control situations and anticipate worst-case scenarios. His self-perception remains conflicted, balancing guilt over past actions with a fragile commitment to his found family in the Agency.