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.

At they walked, Dazai mentioned he can recommend him for a job, but there was a test. Kunikida then came, informing them of a bomber at their agency office. When they went, they saw the bomber having a hostage and while wondering what to do, Dazai beat Kunikida on rock-paper-scissors, so Kunikida had to face the bomber. As the bomber recognized him and was keeping his distance from Kunikida, Dazai realized they need to use someone not from the agency and made Atsushi pretend to be the newspaper boy and a worthless human. As Atsushi was talking to the bomber, he eventually made an opening for Kunikida to disarm him, but while Kunikida was then arguing with Dazai, the bomber managed to free himself and activate the bomb. Having no other choice, Atsushi decided to use his body to lower the explosion damage, but the bomb didn't explode. Dazai then revealed this was the test and job he mentioned. Their president then entered and explained that Dazai had recommended Atsushi and they had to test him. Atsushi wasn't sure he wanted to join them, but as Dazai kept explaining how bad his current life was and how it will go even worse due to his tiger transformations, Atsushi had no choice but to accept.

At a bar, the group were getting to know each other and Dazai made Atsushi guess each member previous job. As he did correctly for all, Dazai revealed he had a reward for his job and that excited Atsushi, who then started randomly guessing professions, but wasn't able to guess right. They received the news of a client being in their office and went to speak with her. Upon seeing the client, Dazai quickly approached her, proposing they suicide together, but Kunikida then took him in the other room and beat him. After hearing her request, Kunikida assigned Atsushi and Junichirou and they went with the client. Dazai remained in the office, listening to music with his headphones, saying he was waiting for a sign. As the client led Atsushi to a dead end alley and revealed she was Port Mafia member, Dazai realized something. A bit later he managed to go where they were and saw Akutagawa and Atsushi fighting, so he used his ability to stop both of their attacks. Dazai then revealed that he planted a listening device on the clients pocket and was listening all of their conversation. She wanted to fight him, but Akutagawa decided it would be better that they leave and revealed that Dazai was ex-Port Mafia member.