Kōyō Ozaki/Synopsis

The Guild Arc
Kōyō speaks to Ōgai Mori, who compliments the beauty of the falling flower petals. Although Kōyō agrees, she draws her blade, saying they'll never drop if they never bloom.

After locating Kyōka Izumi and Atsushi Nakajima, Kōyō calls Kyōka's cellphone, summoning Demon Snow to attack Atsushi. Making her appearance, Kōyō comments how even a pest's blood looks splendid, approaching Kyōka. When Atsushi tries to move, she stomps on his head, telling Kyōka it pained her to let the Agency "blind her" with its light.

She claims she's here to help Kyōka and explains she went to the technician Atsushi mentioned earlier, killing him after modifying the cell phone's connections. Ignoring Kyōka's fear, Kōyō claims she's doing all for Kyōka's protection. When Atsushi shouts that Kyōka isn't returning to the Port Mafia and that her powers are now meant for the Armed Detective Agency, Kōyō sheds a tear. She claims that she 'knew' she shouldn't have waited for Mori's orders to 'save' Kyōka from a "hive of deception and hypocrisy".She embraces Kyōka, saying that the Agency deceived her into thinking their world is filled with 'light' and points out that the Agency will eventually make her use Demon Snow, the ability she loathes so.

She promises to kill anyone who tries using Kyōka for her ability, making Atsushi lash out. Kōyō's Golden Demon lacerates him severely, and Kōyō mocks him, saying he's slow. Golden Demon impales Atsushi against a support beam, and Kōyō apologizes, saying she's only doing her job. Upon seeing Kyōka's terrified expression, she calls Kyōka cute, and that she'd do anything for her, but Mori's orders are to eliminate all Agency detectives. She calls it war, with all Port Mafia leaders in agreement, sans Kyōka because of Kōyō's personal appeal on Kyōka's behalf.

She insists Kyōka must return to the Port Mafia, calling a flower than can only grow in darkness. Kyōka refuses, enraging Kōyō because she sees light in Kyōka's eyes. Blaming Atsushi, she brandishes her blade, threatening Atsushi's life if Kyōka doesn't return. Kyōka relents, agreeing to return with Kōyō, immediately calming the woman. Smiling, she releases Atsushi and says she does everything to Kyōka's sake, and that she'll understand someday.

Escorting Kyōka, Kōyō stops Kyōka from stabbing her with her bare hand, complimenting Kyōka's undetectable intent to kill. However, she's surprised to find out Kyōka stole her cellphone and urges Kyōka not to use it to summon Demon Snow, she tries warning Kyōka what will become of her but is cut off when Demon Snow appears. Both users and their abilities clash, and Kōyō exclaim that Kyōka's blade shows her true self - a demon that kills all in her way. She urges Kyōka to listen to her, saying she knows how she feels.

She expresses her belief in full detail - that flowers born in darkness can only thrive in it, and that the light's heat will burn it in the end, equating this to Kyōka, a 'natural born assassin' trying to be a good person. She recalls her attempt to run from the Port Mafia with a man, who ended up dying, relating the misfortune to Kyōka's current situation, and says the light burned and destroyed her.

Their battle rages on, and Kōyō continues to drill Kyōka's 'darkness' into her mind and heart. Saying Kyōka 'justifies' every massacre to achieve her goals, a nature that will never change. Otherwise, Demon Snow could never be a weapon, not to mention it 'killed' Kyōka's parents. The reveal greatly upsets Kyōka, who gives up resisting and allows Kōyō to take her from the Agency, who promises to protect her. Mafiosi arrive in cars to retrieve them, and Kōyō orders them to shoot Atsushi down.

Kenji Miyazawa and Doppo Kunikida arrive on the scene, saving Atsushi and angering Kōyō, who calls the Agency poison, yelling at them not to show Kyōka their "miasmic light". The clashing sides prepare for all-out war as a result. However, John Steinbeck and Howard Phillips Lovecraft interrupt the skirmish, alongside the arrival of other Guild fellowcrafts. Realizing who they are, Kōyō orders the Mafiosi to fire and kill. Above, Mark Twain uses his ability to fatally injure everyone, including Kōyō.

Saved by Akiko Yosano's ability, Kōyō awakens to Atsushi and Osamu Dazai watching her in the infirmary. Unamused by Dazai's nonchalance, Kōyō asks him if he really thinks restraints can stop her, calling him a traitor that everyone in the Port Mafia wants dead. She asks Atsushi if Kyōka is safe, smiling, and laughing darkly when told Kyōka's gone missing. Dazai stops Atsushi from attacking her, and assures him he'll deal with Kōyō himself. Kōyō refuses to divulge anything to Dazai but complies when Dazai reminds her of how he used to help her torture squad, locking the door and making Kōyō panic.

Dazai 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, Atsushi confronts Kōyō, noting that she's kept in an unlocked room. Kōyō explains the 'lock' is within the mind, thanks to Dazai's interrogation. She guesses Atsushi is here to either finish her off or just happened to enter her room after running away from a mistake on the front lines. Sensing that the latter is correct, Kōyō notes offhand that Dazai always picks the most difficult students. She then asks if Atsushi should be here, instead of helping Dazai.

Atsushi explains Dazai's speaking to a government agent, and Kōyō notes the Special Division would be a valuable aid to the Agency. Atsushi comments she's acting too nonchalant for a mafia executive. Kōyō agrees, summoning Golden Demon and saying she should kill Atsushi and report to Mori with this newfound information. She calmly dismisses Golden Demon, saying that's what she wants to say, but Dazai's prohibited any action on her part. She bashfully says Dazai did something 'unspeakable' to her, which she reveals to be a joke. She clarifies that she and Dazai simply made a deal. She's satisfied as long as Kyōka is found, claiming Kyōka will kill many people during this three-way conflict, forever unable to return to light, just as Kōyō hoped.

Later, Kōyō is freed from the Armed Detective Agency office and returns to Port Mafia, which suffered great casualties after the Guild kidnapped and manipulated Q's ability. She gives Mori a letter straight from Dazai, which urges the Armed Detective Agency and Port Mafia to enter a ceasefire to defeat the Guild.

Following the fall of Moby Dick, Chūya Nakahara treats Kōyō and Mori to one of his prized wines to celebrate victory against the Guild. Mori asks her why she chose not to escape when she was confined in the Agency given that it must have been easy with her ability. Kōyō retorts that it must have been due to the wonderful tea that they served her at the place.

Remembering the deal she and Dazai made before, Kōyō remarks how Dazai's foresight makes him quite a terrifying person. Mori then states that if Kōyō were to leave the mafia, with her powerful skills, nobody might be able to stop her. Kōyō assures him that she has an inept boss to help in reviving the mafia, clarifying that she only hates the previous mafia and that she is now content with the current organization. Mori is gladdened with her response, though he jests that he is only interested in young girls, to which Kōyō asks him to shut up.

The Guild Aftermath Arc
With the Guild's defeat, Kōyō attends a meeting between Mori and Ace. Throughout the meeting, Kōyō openly expresses her disdain and distrust in A, questioning Mori's judgment on allowing him to investigate the mysterious man A's men retrieved.

Afterward, Kōyō stole a transport truck of the Special Division for Unusual Powers, attacking it disguised as a postman. She stole Document No. I-75-81-A, a classified report focused on Kyōka, intended to lessen the severity of her past crimes. 1-75-81 details Demon Snow and how Kyōka's parents truly died. Kōyō leaves it at the cafe when Kyōka isn't present, leaving no trace of her being the deliverer.

Upon her return, Kōyō runs into Kajii, who notes that Kōyō used half a year's worth of funds just to fulfill the task. Smiling, Kōyō says that her 'gift' to Kyōka 's acceptance in the Agency is a cheap price.

Cannibalism Arc
After both Mori and Fukuzawa fall ill to a virus equipped to end after one host kills the other, Kōyō and Chūya deliberate on what action to take within two days before the virus kills the two leaders. Kōyō advises him that fighting against the Agency is playing into the enemy's hands, but Chūya, bent on crushing the mastermind, chooses to face the Agency as soon as possible.

When Jun'ichirō Tanizaki infiltrates Mori's room while incapacitated, Kōyō impales the detective with Golden Demon, asking him to forgive her. With a dark expression, she says that without Mori, the Port Mafia will revert to its former state under the previous mafia boss, the same one that Kōyō attempted to escape from. Tanizaki uses his ability in the battle, but Kōyō states that she can sense his bloodlust, and even though she dislikes the thought of harming Kyōka's allies, she has no choice for the Mafia's sake.

Kyōka eventually saves Tanizaki's life, jumping out the window with him. Mafiosi attempt to shoot them down, but Kōyō orders them not, for they may shoot Kyōka. When they escape, Kōyō approaches the window, watching them with a downcast expression.

After Alexander Pushkin's defeat, Kōyō, alongside the Port Mafia's major mafiosi and the Agency, confront him.