Osamu Dazai/Synopsis

Dazai, Chūya 15 Years Old
Ōgai Mori met Osamu Dazai at 14 years old when he took him in as a patient who had recently attempted suicide. On the same night, Mori murdered the dying former Boss of the Port Mafia whose only aim at that time was more death and destruction with a young Dazai as his only witness. He then told Dazai the cover story: that the Boss died of a serious illness and that his dying wish was for Mori to succeed him. .

Dazai eventually developed a casual relationship with Mori who took him in as a ward. Though the young Dazai was not yet part of the Mafia, Mori had intentions to raise him as his right hand man. During this time, Dazai continued to attempt suicide much to Mori's chagrin. Originally believing them to be "in the same boat", Mori quickly learned he was mistaken when he discovered the young boy was far darker and wiser than he believed.

He and Chūya used to be partners during his days in the Mafia, and together—after they had decimated the entirety of an enemy organization in one night—they became notoriously dubbed as the "Criminal Underworld's Worst Enemy", earning the title Twin Dark (双黒, Sōkoku).

Dazai Osamu and the Dark Era
During the events of Dark Era, Dazai was one of the five Port Mafia Executives. He was the youngest person to become an executive in Port Mafia history (being considered as an executive before he was even 16 ), and a living legend within the mafia as well—to the extent that there is a saying going around the mafia: "The misfortune of Dazai's enemies is to have Dazai as their enemy". In the years he was an Executive, roughly half of the mafia's overall profits could be solely attributed to him, to the extent that Ougai Mori mentioned that, "Given another four or five years, I wouldn't be surprised if Dazai killed me and took over my position." He was also a specialist at torturing people for information, claiming to Kouyou Ozaki that, "There hasn't been a single prisoner who hasn't spilled everything under my interrogation." . He even gave Akutagawa spartan training in using his Ability, and was the one who gave Akutagawa the idea of cutting space to shield himself.

The trigger for Dazai leaving the mafia was his old friend Oda Sakunosuke, a low-ranked mafia member—who was killed in the conflict between Port Mafia and Mimic, orchestrated by Ougai Mori to obtain an official license from the government: proof of the government expressing silent consent of the Port Mafia's existence and its activities as an organization. Already distraught from the knowledge that Ougai intentionally drove Oda Sakunosuke into a suicidal attack against the enemy commander, Dazai took Oda's last words to heart—"I know it makes no difference to you, but please become a good human being. Save the weak, protect the orphaned. I suppose you don't care for either justice or evil...but striving to be a better person is a wonderful thing"—and decided to leave the mafia. He was 18 years old.

It is stated by Akutagawa that, during a mission, he suddenly aborted it and supposedly, defected from the Mafia. Dazai later tracked down Taneda Santouka, who was the acting commander for the Japanese ministry's Special Abilities Division and requested to work in a place where he can help people. Taneda initially thought that Dazai wanted a position in the Special Abilities Division, and so expressed hesitancy, but Dazai said that he does not do well in a place with too many rules. Taneda then recommended the Armed Detective Company to Dazai, on the condition that he'll need at least two years laying low to cleanse his past. Later, it was his friend, Ango Sakaguchi who assisted him with that. It is unknown what he was doing during those two years, but it is mentioned by Dazai himself (though the extent of the truth of the matter is unknown) that before meeting the Armed Detective Company, he was unemployed and unmotivated and drunk in a bar that he used to frequent.

Dazai Osamu's Entrance Exam
At the age of 20, he joined the Armed Detective Company under recommendation from Chief Taneda of the Ministry of Internal Affair's Special Ability Department, and—as a newbie—was entrusted by Fukuzawa Yukichi to Doppo Kunikida, along with administering his entrance exam. Shortly after, they were both assigned to work on the Serial Disappearance of Yokohama's Visitors Case, which Kunikida decided to be Dazai's entrance exam.

Plot
Dazai first appeared floating down a river, apparently trying to commit suicide until he was accidentally found by a starved Atsushi. Afterwards, he and his colleague, Kunikida treated Atsushi to food while listening to his story about how he ended up at the river while hungry. He then introduced himself and Kunikida as part of the Armed Detective Company and they would like to hunt down the tiger that is apparently following Atsushi and terrorizing the areas wherever he went.

At night, while Dazai and Atsushi waited for the tiger to show up, he realized all of the clues that led Atsushi to be confused about the tiger was matching up to be the fact that Atsushi actually was the tiger. As Atsushi lost control of himself and transformed into a full-grown tiger, he was easily subdued by Dazai's ability, No Longer Human, and lost consciousness. Dazai then brought Atsushi back to the dorm where the Agency members are staying. The next day, he called Atsushi for help because he was stuck in an oil barrel trying to commit suicide and gives him the address of the Agency at the same time.