Osamu Dazai.

In terms of physical characteristics, this Dazai is nearly identical to the Dazai from the original universe, with a slim and slender body. His hair is short and black[5], though it appears brown in official art, with bangs that frame his face. Dazai has dark brown, lifeless eyes. Dazai's arms, neck, and left eye are all covered in bandages. He is dressed in a black open trench coat with a long collar, a black tie, a black suit jacket over a white shirt, grey-brown pants, black shoes, and a knee-length maroon scarf draped over his shoulders. When designing him, Sango Harukawa was asked to give him clothes similar to Mori's.

Dazai is much grimmer than his original counterpart, although he still holds his dark, sardonic sense of humor at the worst of times. As the Boss of the Port Mafia, Dazai made many enemies, but the Mafia's influence spans even greater than Mori's influence during his leadership. Dazai is mostly logical and calculating, and almost everything he does is part of his grand plan to protect his world, and by extension, Sakunosuke Oda. However, he is not immune to being swayed by his emotions. Dazai enjoys watching Chuuya struggle, for example, and he tried to befriend Oda despite the fact that their friendship was not meant to exist in this world. This version of Dazai does not have as many meaningful connections with others as he does in the original world. He does trust Atsushi, though. He was also shown to be genuinely happy while talking to Oda in Bar Lupin. Gin described Dazai as "that lonely man" when talking with her brother.

Using No Longer Human, he triggered a singularity and forcibly connected the division between his world and the original world, which makes him able to read the memories of his original counterpart. in his care. Far as seen as fearsome Mafia boss, Dazai already predicted everything from the start in order for Oda to live the life he wanted yet, deep down— he still sought to achieve their friendship in the Bar Lupin scene, he admitted that it was hard, that he did everything for him and know no matter how much and many he tries, their friendship will never exist, he gave his last goodbye to him. In the last phase of his life during the fight of Akutagawa and Atsushi, he fired Atsushi in his job yet he made sure that he will move forward towards the light alongside with Kyōka under the care of Mori Ōgai that is supposed to be deceased. Before his last moments, he trusted Akutagawa and Atsushi to protect the Book, although in fact he doesn't care if the world will cease to exist, it's only where Oda has survived, before jumping to his death, he gave a dreamy smile that everything he planned for has finally come. Yet, in the end, his only regret— he won't be able to read Oda's novel, his only regret before committing suicide.