Kyōka Izumi/Synopsis

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Armed Detective Agency vs. Port Mafia Arc
Kyōka first appears in the city, seemingly waiting for someone. Soon, she spots her target and uses her ability to apprehend Dazai, leaving him in Port Mafia's capture. Afterward, when Atsushi Nakajima was out shopping with Akiko Yosano, they passed by each other without thought. but Atsushi notices her focus on him for a brief moment. Later on, on the train attacked by Motojirou Kajii, she encounters Atsushi once more, and Akutagawa orders her to protect the bomb on the train with her life, thus attacks Atsushi.

After injuring Atsushi severely, she introduced herself as an orphan, listing her likes and dislikes, and confessing to her 35 murders within the span of six months. Their battle continues, but Atsushi soon deflects her. Kyōka shocks him, pointing out the last three victims were a family of three and expresses little emotion. However, after Atsushi urges her to express herself, she wavers. She gives Atsushi the detonator to deactivate the bomb, but it is remotely force-activated after Akutagawa notices the button was pushed. Finally breaking from Port Mafia's influence, she attempts to sacrifice herself by jumping off the train with the bomb. Atsushi saves her and rips the bomb off of her, and the two fall into the river.

She wakes up in the Agency's infirmary. Atsushi and Kunikida persuade her to talk of the mafia's plans with tofu. Atsushi later shows her much of the city she never saw whilst in the mafia, showing her a new side of life. However, at the end of the tour, Kyōka intends to turn herself into the police, aware of her unforgivable crimes, and that it is either imprisonment or mafia's execution awaiting her future.

Atsushi, however, is attacked by Akutagawa, who forces Kyōka to come with him. The three all depart on a cargo ship intended to take Atsushi to the people responsible for his bounty. Nonetheless, Kyōka intends to defy Akutagawa anyway and sets a bomb off to sink the ship with her and Akutagawa with it. Akutagawa overpowers Kyōka and nearly kills her, but Atsushi saves her. After Akutagawa and Atsushi's showdown, Kyōka saves a badly weakened Atsushi from the sinking ship, landing in a rescue boat piloted by Kunikida.

Returning to the Agency, the majority of the detectives celebrate Atsushi and Kyōka's success against Akutagawa, with Yosano and Naomi dressing her up in an outfit. However, when explaining her intentions of joining the Agency, Kunikida points out it won't be easy to deal with her and the crimes she committed, as many forms of authority will want her in their custody. She begs Yukichi Fukuzawa to let her join, saying she wants to prove Akutagawa wrong, that she isn't just a killing machine. After an intense staredown, Fukuzawa hires her.

Dazai arranges for Kyōka to room with Atsushi, both out of convenience and in favor of her safety against any mafia assassins who may attempt to kill her for abandoning them. L

The Guild Arc
Long after Atsushi and Tanizaki's run-in with Lucy Maud Montgomery, Kyōka runs up and hugs Atsushi in relief. Her relief is short-lived. Wearing civilian attire, Port Mafia's boss approaches an unsuspecting Atsushi, praising his efforts against Lucy, and walks away. His sudden appearance terrifies Kyōka to a trembling, speechless state that brings her down to her knees.

The next day, Atsushi hands Kyōka her first work for the Agency, which is to deliver an evidence to a court judge. As they arrive at the location, she is denied entrance, but she tries to resolve it using her own tactical strategy. Atsushi accompanies her to infiltrate the building, though she soon ends up stunning the judge himself, ending in a partial failure. Afterwards, Atsushi treats her to crêpe to cheer her up, later asking her why she still keeps her phone. She explains how it is special to her despite her wish never to activate her ability through it again. However, it suddenly rings, with a voice activating Demon Snow to attack Atsushi.

A woman, Kōyō Ozaki, arrives and tells Kyōka that she has arrived to bring her back to the mafia. Atsushi tells her that Kyōka shall never return to the mafia again, as she now belongs in the Agency. Atsushi activates his ability to attack the woman, only to be stabbed by another demon like Demon Snow. She announces that she has to kill Atsushi and the rest of the Agency upon orders of mafia boss Ōgai Mori. Kyōka then agrees to come with her, though it turns out to be an act to catch Kōyō off-guard.

She snatches Kōyō's phone and activates her ability to battle her, but Kōyō keeps urging her to return to the mafia. She also reminds Kyōka how she is suited for killing, just like how Demon Snow killed her parents. In that moment, mafia reinforcements arrive to finish off Atsushi and retrieve Kyōka and Kōyō. However, Kunikida and Kenji, as well as six Guild members, arrive at the scene, thus inciting a three-way conflict. In the end, the Guild members easily overwhelm all the forces, while Kyōka goes missing.

As Kyōka wanders throughout back alleys and ghettos, she soon comes across Atsushi having been overwhelmed by Francis Scott Key Fitzgerald. The latter recognizes her as a Port Mafia assassin, but she introduces herself as a member of the Agency. She instantly attacks Fitzgerald with her tantō and quickly rescues Atsushi to escape.

Hiding from Fitzgerald, Kyōka informs Atsushi that she has called the police with a false report to arrive at their location and to prevent the Guild from performing any rash actions. Atsushi wonders why Kyōka came back instead of remaining incognito and staying out of the war, but she tells him that she belongs to the Agency and promises to fight for it, even if she needs to resort to her inner monstrosity. They soon approach the two policemen nearby to seek aid. As one of the two officers is notified that a young wanted serial killer in kimono is spotted in the area, he grows suspicious of Kyōka. She immediately slashes the officer out of stimulus, but Atsushi quickly stops her from attacking the second officer and grabs her to escape.

They soon encounter Fitzgerald and his companion, having anticipated the situation with the help of their tactician. As Kyōka and Atsushi are instantly and severely shot from above by Mark Twain, Fitzgerald grabs Atsushi and takes him to Moby Dick summoned by Herman. Amidst her pleas not to take Atsushi away, Fitzgerald advises her that saving others does not suit her, to which she weeps and wonders why she is given hope if such were the case. As the Guild departs with Atsushi, the military police surround and arrest Kyōka, while she imparts her goodbye to the one who showed her the light.

With Kyōka confined in a UAV, Dazai contacts her and gives her instructions on how to land the aircraft after negotiating with the Special Division for Unusual Powers, but Kyōka declines and has given up hope for herself. 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 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. Kyōka soon hears about the hacking of Moby Dick; she communicates with them and decides to collide the UAV with the ship to divert its course. Atsushi asks her to escape, but, unable to, she instead thanks him and says her goodbye.

Dazai and Fukuzawa soon approach Atsushi and inform him that Kyōka has successfully passed her entrance exam the moment she has decided to sacrifice herself and 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. Atsushi then rushes to Kyōka and embraces her out of relief, while the other Agency members proudly look on to them from a distance.

Kyōka is later accompanied by Atsushi to the Agency office, where the Agency welcome their newest member. Kunikida later approaches her and Atsushi and scoldingly imparts them with various pieces of advice as their superior, especially not to tarnish the Agency's name and do anything they will regret. Yosano soon urges him to simply tell them what he wished to say, to which he complements them for a job well done in saving Yokohama. Kyōka later calls for Naomi and Haruno per Atsushi's request so he can apologize to them.

The Guild Aftermath Arc
While eating in her and Atsushi's dorm, Kyōka shows her distrust towards Demon Snow, which independently assists her with some work. The next day, they head to Café Uzumaki, where they are welcomed by an annoyed Lucy. After the latter voices her grudge towards Atsushi, she hands them a dispatch specifically for Kyōka, which is to deliver a briefcase to a client. As they head to a quay, they split up and search for the boat containing the briefcase. Later, Kyōka spots Atsushi on the water after he is kicked out of Lucy's dimension. After obtaining the document that reports on the death of Kyōka's parents, they realize that Demon Snow – which Kyōka long thought killed her parents – is actually a sign of her parents' love for her.

Kyōka and Atsushi try out Lucy's coffee, which she finds average, unlike Atsushi who compliments it. She remarks that, if such coffee costs the same as the manager's, their customers will not go along with it.

Plot
Kyōka appears behind the assassin's holding her dagger against his neck. She then asks Atsushi if she can kill him, the reaper says there's no need, explaining the boss needs one of them spared to tell their higher-ups that their plans were foiled easily. The assassin recognizes the girl as Kyōka Izumi, a killer of 35 people. Confused on how she is still with the mafia after the rumors of her betrayal, Kyōka replies she did indeed betray the mafia, but came back for the reaper.

As Atsushi and Kyōka walk through the streets, Atsushi suddenly kneels down, wincing in pain with blood dripping down his neck. Revealing a thorned collar around his neck, Kyōka advises Atsushi to take it off, but Atsushi refuses. The Black Lizard comes across them, Atsushi thanks them for guarding the area. Hirotsu tells Atsushi to seek treatment at the base and report to the boss afterwards, going on to explain how they knew about the bombs and had Kyōka take care of them. The two then left.