Osamu Dazai/Synopsis

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Fifteen Arc
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,).

Dark Era Arc
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 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 starved 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.

At night, Dazai wanders the streets and encounters a girl in kimono who tucks on his sleeve. She activates her ability and soon takes him to their base. Akutagawa later visits his chained ex-mentor, and the two exchange hostile pleasantries. Dazai states how he has been a chore to train in the past and claims that his new subordinate is a lot better than Akutagawa himself, provoking the latter.

Dazai's former partner, Chūya Nakahara, later visits him in his cell and asks his motive for getting himself captured by the mafia. He tells him that he has simply been caught and is awaiting execution, but Chūya thinks otherwise, knowing how elusive Dazai is to be easily captured. Chūya then removes Dazai's shackles and challenges him to a duel. As Dazai reveals how he has already unchained himself, he quips if Chūya can ever foil his plan, upon which the mafioso charges at him.

Chūya easily overwhelms Dazai in physical combat and asks him again to state his true motive for capture, to which Dazai admits that it is all for Atsushi. He then tells Chūya that he has sent a letter to the executives threatening to spill the mafia's secrets should Dazai get killed. As Chūya soon realizes Dazai's setup, he ends up telling Dazai where the mafia's records – that might reveal who placed the bounty on Atsushi – are kept. He later sneaks in the archive room and discovers the one responsible for Atsushi's bounty, much to his surprise.

The Guild Arc
After slacking off following his capture, Dazai soon returns to the Agency. He also advises a panicking Atsushi that it is his job to look after his new roommate Kyōka, which Atsushi accepts. He then states that the one who placed the bounty on Atsushi is a North American gifted organization known as the Guild. Tanizaki bursts in and tells them that the Guild has just arrived at their office. Their leader, Francis Scott Key Fitzgerald, asks to purchase the Agency from Fukuzawa, an offer which the President rejects.

The following morning, the Agency are unsettled by the possible actions undertaken by the Guild, including how a mafia-affiliated building vanished overnight and how Kenji Miyazawa has turned missing after seeing the visitors out the office the previous day. Dazai and Kunikida convene with the President, while Tanizaki and Atsushi head out to search for Kenji.

The Agency soon inadvertently enters in a three-way conflict with the Port Mafia and the Guild. Mafia executive Kōyō Ozaki is captured by the Agency, whom Dazai and Atsushi later converse with in the Agency infirmary. Atsushi gets angry at her for causing Kyōka's disappearance after the Guild's surprise attack on them and transforms his arm into a tiger limb, but Dazai quickly stops him from rampaging by nullifying his ability. Asking Atsushi to leave the matter to his hands, Dazai threateningly turns to Kōyō to settle the issue.

Dazai then makes a deal with her in an effort to save Kyōka, which includes having to get her arrested, after which he shall have a judicial arrangement with the Special Division for Unusual Powers. He claims that such way shall allow Kyōka to enter the Agency and save her in the process. Kōyō deems it impossible, but Dazai assures her that the former mafia that she hated is now long gone; the current mafia is now different, and Dazai can thus assure that her fate will not befall Kyōka. As Kōyō reminisces that the man she loved before promised her to take her into the radiant world, Dazai urges her to do the same thing for Kyōka and take her out into the light herself.

Later, the Agency convenes at the Bankoudou Hall, where Dazai explains their strategy. To make up for their numbers, they shall split into various teams: one defense team and two offense teams. Dazai is paired with Atsushi as one offense team; Tanizaki is partnered with Kunikida as another offense team; and Fukuzawa, Ranpo, Yosano, and Kenji comprise the defense team. Dazai then finishes with their main objective which is to keep their base hidden as possible, after which Fukuzawa declares that they must survive the three-way ability war.

Dazai and Atsushi later await Naomi's and Kirako Haruno's passenger train from the Guild's hitmen; Dazai teases a dog with some treats which he also eats himself. Atsushi worries how the Agency can pull through the conflict, though Dazai thinks that they can turn the tide by about three hundred ways. Nonetheless, Dazai recognizes the mafia's most advantageous position compared to their least advantageous, considering Mori's logical and cunning calculations. Since the clerks have now let down their guard, Dazai reckons that Mori shall exploit such vulnerability.

Dazai's stomach later aches possibly from the dog treats he ate. He then leaves Atsushi behind and heads out. Knowing he is being tailed, Dazai urges the mafioso following him to come out. He then greets Gin and Higuchi, asking why they have come considering that there are too few of them for a possible assassination attempt on his life. Higuchi delivers a message from Mori, which invites Dazai to return to the mafia as an executive.

Dazai merely laughs at the invitation, upon which Higuchi remarks how his mafia records prove his sadistic tendencies that surpass possibly anyone else. Dazai retorts that people do change, citing Gin as an example from being a little girl when he last saw her, which gets Gin flustered. Though Dazai turns down Mori's offer, he wonders why Mori would waste manpower on him. Higuchi informs him that they are to protect him after Mori has freed Q from captivity, leaving Dazai aghast. Initially in disbelief, Dazai soon realizes that Atsushi may already be in peril.

As Dazai returns, he finds the cursed Atsushi having attacked Haruno and now strangling Naomi, asking him to come to his senses. As Atsushi grasps the situation, Dazai grabs Q's ripped doll and nullifies the ability to end the curse. Aboard the departing train, Q sends their regards to Dazai and threatens to kill him for locking them up before, while Dazai assures them that he shall hollow out their heart in the future rather than mere imprisonment. As Atsushi undergoes a breakdown, Dazai slaps him and lectures him not to pity himself, lest his life will be an unceasing nightmare. As Dazai realizes that he must also dirty his hands under such circumstances, Dazai decides to drag the government into the war for their counterattack.

Soon after, Dazai meets with Ango, the Assistant Counselor of the Special Division. He cheerfully greets Ango, only to snatch the latter's gun to point it at him out of grudge. Dazai wonders how he thinks he has already forgiven him, only for Ango to remind him that he is the one who cleaned his ledger. Knowing that Ango has anticipated the situation and that the gun is therefore not loaded, Dazai returns it to Ango and suggests they go for a ride.

Dazai briefs Ango on the Guild's recent movements in Yokohama, but Ango and the Special Division are actually aware of their activities. He explains that the Guild is a secret society whose members all have diplomat rights and cannot therefore be held in legal custody. As a result, the Special Division cannot make any rash actions so as not to disrupt any connections with the Guild. Ango then asks Dazai to run and to tell his comrades that they are in danger, after which a vehicle rams into their car.

Later, Dazai returns to the Agency after Atsushi gets abducted by the Guild. At some point, he presumably lines up some smokescreen capsules down the streets as a precautionary measure and watches the sky on the office balcony. While waiting, Kunikida approaches him and shows a handprint blotch on his neck, asking Dazai what the situation is all about. He and the other Agency members soon restrain Kunikida, who later tells Dazai that the curse has begun.

As Atsushi jumps off the Moby Dick with Q's doll, Dazai eventually intervenes his fight with Mark Twain. Approaching Atsushi, Dazai nullifies the curse by touching the doll and sets off a saturation chaff smokescreen that emanates from the capsules, jamming Twain's infrared sensors and radar.

Dazai and Atsushi retreat to a nearby nook. Though they have stopped the cursed rampage, Dazai tells Atsushi that Q is still in the Guild's confinement and can launch the same attack how many times they want. He adds that the Special Division is currently frozen and cannot therefore collaborate with them. Recalling the quote he once read in a book – that the head may err but never the blood – Atsushi recommends a solution in fighting the Guild: forming an alliance with the Port Mafia. As Atsushi conveys his suggestion to the President, Fukuzawa soon tasks Dazai to set up a secret meeting with the mafia.

The following morning, Dazai is quite unenthusiastic and tries to get cheered up by Kunikida, who wonders why he keeps complaining despite recently saving Yokohama together with Atsushi. Dazai explains how he feels off with the next assignment given to him by the President regarding Atsushi's plan on their next course of action. Fukuzawa enters, and Dazai updates him about the arrangement of the secret meeting with Mori. Dazai thinks that Mori shall accept their proposal, as he sees it to be an opportunity to assassinate Fukuzawa. The latter then takes his leave, while Kunikida gets surprised on his assignment, more so because he is the one planning the meeting. Dazai informs him that he has been tasked with such mission because he is formerly from the mafia, causing Kunikida to collapse out of shock.

Later, Dazai welcomes Mori and his entourage to commence the secret meeting. In the end, the two parties soon arrive at the conclusion that they cannot have an alliance. As the mafia leaves, Mori tells Dazai that his invitation to return as a mafia executive still stands, to which Dazai quips if the boss is quite afraid that he might kill him soon like how Mori killed his predecessor. In the end, Dazai states how he is also against collaborating with the mafia.

At night, Dazai heads to Q's location, where he gets cornered by Steinbeck, Lovecraft, and various Guild operatives. Lovecraft gets struck with a boulder hurled by Chūya, much to Dazai's irritation upon realizing they have to work together for their counterattack. Steinbeck summons his grapevines, but Dazai easily nullifies his ability, after which Chūya subdues Steinbeck.

Afterwards, Dazai and Chūya retrieve Q and their doll, with Dazai keeping Q alive as insurance that the mafia does not kill him due to his nullification ability. They soon encounter Lovecraft, whose attacks Dazai cannot nullify; he concludes that his tentacles are not an ability. As Lovecraft transforms into a humongous creature, Dazai urges Chūya to activate Corruption, which the mafioso begrudgingly uses.

Nearby, Dazai asks Steinbeck at knifepoint regarding Lovecraft's true form, but Steinbeck refuses to spill any intel to him. Nonetheless, Dazai sees through his words and detonates a bomb he has embedded earlier in Lovecraft's core, having Chūya finish him off. As Corruption keeps Chūya on a rampage, Dazai nullifies his ability and asks him to rest. Before he falls unconscious, Chūya asks Dazai to bring him back to base. In the end, Dazai leaves him behind and returns on his own.

Later, as Dazai and Ranpo plan their next action after retrieving intel on Moby Dick, Dazai tasks Atsushi to solo infiltrate the aerial fortress. He explains that Atsushi is a combat-type and high-speed ability user who has already been aboard the aircraft and would only be captured rather than killed if ever he gets caught, making him the most suitable choice for the mission. Atsushi eventually accepts the task.

Dazai visits the hospitalized Ango, cheerfully informing him that Yosano has agreed to heal his injuries following their car crash. Ango sees through his scheme and asks what he wants in return, to which Dazai asks for his intervention in freeing a captured Agency subordinate. Ango admits that the Special Division can pardon a killer of 35 people, provided that she is an official member of the Agency. He also hints that they can offer their services to the Agency in purging the Guild. As Dazai takes his leave, Ango asks him why only his airbags malfunctioned in the car crash, but Dazai merely responds with a dark smile.

As Atsushi successfully infiltrates the aircraft, he informs Dazai through the comms that the ship is devoid of its crew, except for Herman Melville, from whom Dazai hears about the Guild's next plan of crashing Moby Dick to Yokohama. Atsushi urges they abort the mission, but Dazai tells him that he must stop the descent himself, being the one aboard Moby Dick. Dazai is soon contacted by Mori, who tells him that a report he has received from his subordinate has compelled him to overlook their mutual agreement to a temporary ceasefire, having sent Akutagawa onboard Moby Dick as well. Realizing that Akutagawa has now encountered Atsushi, Dazai instructs Atsushi on how to throw off Akutagawa from their path, which lets Atsushi successfully escape.

Dazai then contacts Kyōka who is confined in a UAV that isolates dangerous ability users. He gives her instructions on how to land the aircraft after negotiating with the Special Division, but she refuses to follow and has given up hope. Dazai admits that the Agency has no reason to save her given that she is not an official member, whose entrance exam she thinks she will fail either way. Dazai then wonders if she thinks that a former killer cannot become a good person, since he himself does not believe such case.

Dazai understands how she thinks that she is not suited for the Agency, but he dismisses the thought and lectures her on possibilities, just like Atsushi, who was previously regarded as a beast, who took her in, and who is now fighting with his life on the line for the sake of others. Knowing that Kyōka is at a loss, Dazai convinces her that it is but human to lose one's way and dash recklessly along life like stray dogs rather than trying to find purpose in life or what they fight for. Thus, he convinces her that it is but human to lose one's way and dash recklessly along life like stray dogs rather than trying to find purpose in life or what they fight for. Later, Dazai is updated by Atsushi about the hacking of Moby Dick; eventually, Kyōka collides the UAV she is in with Moby Dick and diverts the ship's course.

As Atsushi is distraught with Kyōka's fate, Dazai and Fukuzawa arrive to tell him that Kyōka has passed her Agency entrance exam the moment she has decided to save Yokohama. As a result, the President's inhibiting ability that only works on his Agency subordinates has activated and allowed Kyōka to freely control her ability, enabling her to have Demon Snow cut off her shackles and escape in time, much to Atsushi's relief. Akutagawa calls on to Dazai to prove and show him his improvement, only for Dazai to recognize how he has grown strong, causing Akutagawa to faint.

After the Guild war, Dazai meets up with Hirotsu at an art museum. They observe an artwork that Dazai deems to be quite strange and amicably reminisce on Dazai's past painting that Elise thought was cursed. Dazai then thanks Hirotsu's great help in the war. Though all that Hirotsu did was spill the Moby Dick infiltration plan to Higuchi, Dazai claims it to be sufficient for Akutagawa to learn about it from Higuchi and charge in on his own.

Hirotsu asks his aim in bringing Atsushi and Akutagawa together; Dazai tells him that he wishes to test if the two can become a new generation of Twin Dark in fighting the real disaster that lurks beneath. He admits that he might not anticipate the next actions of the demon he once met in the past, though he knows that the latter must have already started moving. In the anime, Hirotsu further asks Dazai why he cherishes Yokohama so much, to which Dazai responds that his old friend once told him to become a good man if both sides are the same. As he states how saving people has made him feel better, Hirotsu remarks and wonders if it is flawless.

Plot
Dazai appears before the injured Akutagawa after he killed the 6 armed males who attack them. He then responds to Akutagawa's question as to if he was going to recruit him into his organization but he changed his mind. Dazai then describes Akutagawa as nothing but an unintelligent dangerous animal, Akutagawa retaliates by attacking him with Rashomon but is easily beaten down. He then leaves Akutagawa unable to move, explaining he will take care of his sister until Akutagawa is strong enough to beat him.