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 Ōgai 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 Kōyō 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 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 Mori 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.

It is stated by Akutagawa that, during a mission, he suddenly aborted it and supposedly, defected from the Mafia. Dazai later tracked down Taneda, 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 Agency 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 Agency, he was unemployed and unmotivated and drunk in a bar that he used to frequent.

Azure Messenger Arc
Dazai had eaten some weird mushrooms following his guide book for suicide and acted weird in front of Atsushi and Kunikida. Following an anonymous tip, they managed to save a kidnapped woman Nobuko Sasaki, but few others died from gas. On the next day, Dazai was messing with Kunikida about his ideal girl and glasses, when Kunikida realized something. He called their taxi informant and revealed he was the kidnapper, who confessed but revealed he was being forced and wanted protection. Following his information, they went to a place have they received a request from the anonymous tipper about disarming a bomb.

On the next day, Dazai was speaking with Sasaki in a cafe, where Kunikida and Atsushi later came. As they discussed the bomb, Dazai asked Sasaki about her opinion and she suggested the culprit was related to Azure King. Dazai then tried hooking Kunikida with S Sasaki as he was her type and even revealed Kunikida's type to Sasaki, but she thought it was too much. They received news from Ranpo, as he as done with his case and he told them where the bomb was. Heading towards it, the bomb squad had already found it but weren't able to disarm it. The three of them received calls from Ranpo, Rokuzō and the president.

With the new information, they went towards a former defense facility, where they got attacked. Kunikida ended up fighting a man who could put a number on Kunikida and freely control him. Dazai and Atsushi looked at how to disarm the bomb but found out they need a key to deactivate it. A large man then came and started beating them. As they didn't stand a chance, Kunikida saw Dazai and managed to switch places with him, taking out the man, while Dazai neutralized the other man's ability. Kunikida then recreated the key needed to stop the bomb and they managed to disarm it

On the next day, Dazai had figured it all out and called Kunikida to the abandoned hospital, where he said the culprit will also come after he receives his message. As Rokuzō came, Kunikida thought he was the culprit, but Dazai explained he probably hacked his email which he used to call the culprit. Sasaki then came and shot Rokuzō, revealing that the Azure King was her boyfriend and that she was the Azure Messenger. As Dazai couldn't shoot her, he dropped his gun, who was picked by the dying Rokuzō and he ended up killing her. Dazai mentioned that Kunikida ideal may one day lead him on the path of the Azure King, but Kunikida assured him, he will not turn like him.

Armed Detective Agency vs. Port Mafia Arc
Dazai first appears floating down a river, apparently trying to commit suicide until he was accidentally found by a starves Atsushi. Afterward, he and his colleague Kunikida treat Atsushi to food while listening to his story about how he ends up at the river while hungry. He then introduces himself and Kunikida as part of the Armed Detective Agency 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 wait for the tiger to show up, he realizes 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 transforms 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 calls 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 mentions 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 recognizes him and was keeping his distance from Kunikida, Dazai realizes 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 manages to free himself and activate the bomb.

Having no other choice, Atsushi decides to use his body to lower the explosion damage, but the bomb didn't explode. Dazai then reveals this was the test and the job he mentioned. Their president then enters and explains 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 cafe, the group was getting to know each other and Dazai made Atsushi guess each member previous job. As he did correctly for all, Dazai reveals he had a reward for his job and that excited Atsushi, who then starts randomly guessing professions, but wasn't able to guess right. They receive the news of a client is in their office and went to speak with her. Upon seeing the client, Dazai quickly approaches her, proposing their suicide together, but Kunikida then took him in the other room and beat him. After hearing her request, Kunikida assigns Atsushi and Tanizaki and they went with the client. Dazai remains 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 reveals she was Port Mafia member, Dazai realizes something. A bit later he manages to go where they were and saw Akutagawa and Atsushi fighting, so he uses his ability to stop both of their attacks. Dazai then reveals that he planted a listening device on the client's pocket and was listening to all of their conversations. She wanted to fight him, but Akutagawa decided it would be better that they leave and reveals that Dazai was ex-Port Mafia member.

Ranpo and Atsushi were assigned at a murder case at the river, the net then caught something and as they pulled it out, they saw it was Dazai, who explains he was just floating himself and not attempting suicide, as he wants to now make double suicide with a beautiful woman. As he saw the dead body of an attractive woman, he regrets that she died and that she could have suicided with him, but was sure that Ranpo will solve her case. Ranpo was then allowed to solve the case and realizes her killer was another police officers. After solving the case and going back towards the agency, Dazai explains that Ranpo wasn't gifted and he didn't have the ability. It was the first time he saw Ranpo using his ability, so he touched his hair to neutralize it, but since it didn't have an effect, he knew Ranpo wasn't gifted. Dazai explains that he himself had already solved half of the case, but likely Ranpo noticed a few more details to completely solve it.