Kyouka Izumi

Kyouka Izumi (泉 鏡花 Izumi Kyōka) is a defected agent of the Port Mafia in Bungou Stray Dogs. Her name is a gender-bent version of the Japanese author Kyōtarō Izumi. Her ability, Demon Snow, is a reference to his work, Demon Pond.

History
Kyouka's father was a former secret agent of the Japanese government. After 'causing an incident', he resigned and became a freelancer assassin. His wife and partner in crime owned the powerful ability named Demon Snow. They were very successful in their assassinations but created significant enemies. Kyouka's mother gifted her a phone and told her to never let go of it.

Kyouka was completely unaware of her parent's work and lived a relatively normal and happy life.

That ended when a group of assassins with an unknown affiliation attacked Kyouka's parents in their house. Most of the assassins were easily disposed of through her father's firearm proficiency and her mother's special ability. However, the assassins apparently also had an ability user and they were able to control Kyouka's father's body through blood manipulation. He was manipulated into attacking Kyouka, all the while begging to be killed before he killed his daughter. Demon Snow, under the control of her mother, successfully killed her father before he killed Kyouka. Her mother was then also put under manipulation, and she ordered Demon Snow to protect Kyouka from herself, and Demon Snow killed Kyouka's mother. Getting what they came for, the assassins left Kyouka alive.

Somehow, in the midst of that incident, Kyouka's mother transferred the ability Demon Snow into Kyouka. The transfer was imperfect, however, and as such could only be controlled through the medium of Kyouka's phone that she kept in memorial of her mother.

Kyouka, not knowing about the assassins, her mother's ability, or even that her parents were being manipulated, falsely interpreted the incident as her own power manifesting for the first time and going out of control, killing her parents.

As an orphaned, helpless young girl with a powerful ability, she was easily taken by the Port Mafia. She was under the care of Kōyō Ozaki, a Port Mafia agent with an almost identical ability. Kōyō protected, cared for, and loved Kyouka like her daughter. While working for the Port Mafia, she was used as a powerful assassin and killed a total of 35 people in 6 months. The stress of killing this amount of people affected her mentally, and she stopped displaying emotions the same way a normal kid should. She was turned into a killing machine.

Her final mission as a mafia agent involved her assisting Motojiro Kajii in a bombing of a public transportation train (chapter 7 of the manga). Atsushi and Yosano, two Armed Detective Agency detectives, were on the train at the same time. In a public skirmish, Atsushi confronted Kyouka on an empty sector of the train while Yosano confronted Motojiro similarly on the other side of the train. During the standoff phase of the confrontation, Kyouka received a call from Akutagawa ordering her to protect the bomb with her life and kill Atsushi. She emotionlessly attacks Atsushi with Demon Snow at this order.

Atsushi suffers the first two nearly lethal blows without fighting back. All he does is ask her why she, as a young teenage girl, is being so homicidal. Kyouka responds only with a monotone and emotionless monologue giving her name, her likes (Tofu and Rabbits), her dislikes (Thunder and Dogs), and that she has killed 35 people. Atsushi, in a sudden spark of determination, takes his hybrid form, charges forward, dodges Demon Snow, and holds his clawed fingers by Kyouka's throat, threatening her. Atsushi orders her to deactivate her ability and tell him where the bomb is. She responds with another emotionless monologue where she mentions her most recent kills, a family of three including a child. She then opens her kimono revealing that she is wearing the high explosive. Atsushi questions her again, not believing that a young girl was capable of being this emotionless. He tells her to question if this is what she really wants.

Yosano, having already defeated Motojiro and tortured him for information, used the train's announcement terminal to inform Atsushi that the bomb was going to activate in a few seconds but could be defused with an emergency stop button. Atsushi, assuming Kyouka had it, demands Kyouka gives it to him, to which she wordlessly complies. He presses the button, but it turns out it activated a shorter timer on the bomb. This surprises Kyouka as much as it surprised Atsushi, her face making the first emotional expression since the start of this mission. Akutagawa's voice comes on the phone telling her not to take the bomb off, but just to take the train with her and show everyone how scary the Port Mafia is. Atsushi, in a panic, tells her to take off the bomb, to which she replies "It's too late!" and stands at the broken doors of the train (that was now on a bridge over a body of water). She makes her third short monologue here: "My name is Kyouka. I have killed 35 people. I don't want to kill anymore!" She then jumps off the train to take the bomb with her. On the way down, Atsushi jumps to her, rips the bomb off of her, and protects her from the explosion, saving both the train and Kyouka.

Personality
Kyouka, during her Port Mafia months, has an extraordinarily detached character. She shows no emotion as she kidnaps Dazai or attacks Atsushi. Her detached figure is reminiscent of a doll or a mannequin. After being saved by Atsushi, she has shown her more normal, childlike personality. She takes a liking to Atsushi and enjoys the various arcade games and amusement park rides in the city. However, she still has a stoic mindset as shown when she willingly turns herself in to the police, knowing she would most likely be put on death's row.

Even after being saved, she is still rather homicidal and is likely to resort to violence. While being threatened by an innocent cop, she wounds the cop, potentially lethally, in defense when nonlethal incapacitation was possible. This mindset is what kept her from officially joining the Armed Detective Agency early on. Despite that, she still has an altruistic mindset and has twice shown willingness to sacrifice her life for the greater good. Her second sacrifice is what got her into the Armed Detective Agency officially.

Kyouka has taken a great liking to Atsushi, staying by his side whenever she can. The only place she feels safe is around Atsushi and because of that she has been sharing a room with him.

Abilities

 * Demon Snow(夜叉白雪Yasha Shirayuki) is an ability that manifests a large, substantial, sword-wielding, humanoid phantom. It can only be controlled by calling Kyouka's phone due to the botched transferral of power from her mother. The demon has an immense striking force and is extremely fast. However, it has been shown to have limited lifting strength as it couldn't free itself when pinned down by daggers to a wooden board.
 * Acrobatics: Kyouka has shown immense physical capabilities, making superhuman leaps and slicing people with immense force, even without her ability.
 * Sword Play: Kyouka is skilled with a sword.