Jun'ichirō Tanizaki/Synopsis

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Armed Detective Agency vs. Port Mafia Arc
Tanizaki poses as a hostile enemy with a grudge against the Agency, threatening to bomb everyone and having his sister pose as a hostage. This charade acts as Atsushi Nakajima's entrance exam into the Agency.

After the incident, Tanizaki apologized to Atsushi, saying that even if it was for Atsushi's exam, he was still rude to him. This fact shows off how much of a caring person Tanizaki can be. It was also when he had told Atsushi that he too worked as an assistant of sorts at the agency.

He and Naomi later attend Atsushi's first major mission, assisting a woman claiming to be stalked. The mission ends in an ambush orchestrated by Port Mafia, and the woman violently shoots at the trio. Naomi saves Tanizaki's life by using herself as a shield. Naomi's dire state makes Tanizaki snap, and he uses Light Snow to counter Higuchi's assault.

Just as he's about to crush her neck, Ryunosuke Akutagawa severely injures Tanizaki, leaving Atsushi on his own against Port Mafia's hellhound. Later, Dazai intervenes and cuts the fight, and Tanizaki makes a full recovery with the help of Akiko Yosano's ability.

While the Agency works on a request from a ministry officer, the Agency learns about Atsushi's kidnapping by the mafia. Though Tanizaki suggests they save him, Ranpo reasons out why they have no reason to do so. Worried for Atsushi, Naomi brings in the President himself to make a decision. In the end, Fukuzawa puts all work on hold and orders the Agency to help bring back Atsushi within three hours.

As they find the corporation that owns the vehicle that captured Atsushi, Tanizaki investigates their base, only to find all crew members slaughtered by Akutagawa. In the end, after Atsushi manages to defeat the latter, Tanizaki is among the members welcoming Kyōka Izumi to their team.

The Guild Arc
Following Port Mafia's failed kidnapping, the Guild makes its move to obtain Atsushi themselves. Once more accompanying Atsushi with his sister to investigate Kenji Miyazawa's disappearance, the trio breaks when Lucy Maud Montgomery's ability traps Naomi.

A panicked Tanizaki tracks Lucy down, but she quickly traps much of the public in her dimension. Despite his ruthless desire to save Naomi and Kenji, Tanizaki also falls victim to Lucy's ability. However, Light Snow creates a false door and tricks Lucy. Together with Atsushi, Tanizaki succeeds in freeing Naomi and Kenji along with the rest of the imprisoned civilians in the dimension.

After the incident, Tanizaki serves some baumkuchen to Naomi for breakfast. As Tanizaki reads the metropolitan police's records on Kyōka, he tells Naomi on how her parents were murdered.

The Agency later convenes at the Bankoudou Hall, where Dazai explains their strategy against the mafia and the Guild. To make up for their numbers, they shall split into various teams: one defense team and two offense teams. Tanizaki is partnered with Kunikida as one offense team; Dazai is paired with Atsushi 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.

Eventually, the mafia comes up with a strategy of using the Agency clerks – Naomi and Kirako Haruno – as bait for the Guild, particularly for Steinbeck and Lovecraft. Upon learning of their tactic from Chūya Nakahara, Fukuzawa contacts Kunikida about the matter. Upon the President's advice, Kunikida then informs Tanizaki that open conflict with the Guild is imminent, heading out afterwards to save the clerks on time.

As they encounter the two Guild members on the road having trapped the clerks' escape vehicle, Tanizaki activates Light Snow for cover, allowing Kunikida to shoot Lovecraft and Steinbeck from behind. As Steinbeck inadvertently loosens his grapevines' grip on the clerks' vehicle, Tanizaki rescues the clerks and directs them to flee to a nearby passenger train.

After Tanizaki helps Naomi and Haruno escape from the car, Kunikida tries to buy some time for the clerks, only for Steinbeck to rid him of his pistol and restrain him. However, Kunikida manages to gain the upper hand by manifesting another pistol through a pre-torn page from his notebook, instantly subduing Steinbeck.

To their shock, Lovecraft stands up, seemingly unscathed from his earlier gunshots. Kunikida asks Tanizaki to activate Light Snow immediately, but Lovecraft's tentacles capture the two detectives at once. While they are restrained, Steinbeck locates the two clerks through his grapevines and successfully prevents their escape. Kunikida questions their means, to which Steinbeck responds how moralists like them would never understand their work.

Provoked with how the Guild puts Naomi in harm's way, Tanizaki retorts that he shall gladly burn the world and discard his morals for Naomi's sake; he uses Light Snow on an oncoming truck driver to have it crash onto Steinbeck and Lovecraft. Upon doing so, Tanizaki and Kunikida are released from their restraints and successfully retreat to the woods.

After Atsushi and Dazai stop the rampage of the cursed Yokohama citizens, Tanizaki accompanies Fukuzawa to a secret meeting with Mori and his entourage. As the two leaders briefly clash, Tanizaki reveals Fukuzawa to have been his projection. The real President later arrives, and the meeting concludes with the two parties declining to form an alliance.

Later on, Tanizaki uses Light Snow to hide the helicopter he pilots to give Atsushi cover so he can board Moby Dick and defeat Fitzgerald once and for all.

Cannibalism Arc
After the Guild incident, Fyodor makes his move using a mysterious ability-user assassin that allows Pushkin to infect both Mori and Fukuzawa. Once it becomes unavoidable that one of the infected must die to save the other, the Port Mafia and Agency enter yet another heated war to save their respective leaders. Tanizaki personally goes to where Mori lies unconscious and helpless thanks to the disease, but Koyo stops him and nearly injures him. Soon the struggle seemingly ends in Port Mafia's favor, having caught Tanizaki, but Tanizaki's spirit remains unbroken and unfazed, unsettling the executive members a great deal.

Plot
Tanizaki poses as a bomber holding a hostage inside the detective agency. He threatens to detonate the bombs and Akutagawa tells him to do it, acting on his threat, the bomber pushes the button. He was then pinned on the wall by Akutagawa after destroying the bomb. Later that day, he apologizes to Akutagawa and introduces themselves as Jun'ichirō Tanizaki and the hostage, his sister Naomi. Tanizaki and Kunikida have a mental conversation, thinking Akutagawa seems angry with Tanizaki as he is looking at him angrily. Tanizaki is asked by Akutagawa if Naomi is really his sister to which he confirms it. While at the cafe, it's up to Tanizaki and Kunikida to asses whether or not Akutagawa is truly able to take on the job since he has only been preliminarily accepted.