Izumi, Kyouka

Ability: Demon Snow; allows her to materialize a sword-wielding phantom, known as Demon Snow, which only follows orders coming from a mobile phone she always carries on her person. It appears that it only listens to orders from another person on the other line and it does not comply to Kyouka's own wishes.

Though after joining the Armed Detective Agency this problem was fixed by Fukuzawa Yukichi which enables her to control her own ability and use it normally. In reality, Kyouka is not the original wielder of Demon Snow, but instead, her mother was. In order to protect Kyouka, in her last moments, Kyouka's mother transferred her ability Demon Snow to Kyouka , but because of the suddenness at the time, control of the demon was transferred to Kyouka's cell phone instead of Kyouka herself.

Assassin for the Port Mafia, Kyouka's personality is similar to that of a doll. She lacks all emotions and is ruthless when she is ordered to perform a duty.

After being exposed to Atsushi and the real world, Izumi reverts back to a young teenager. She is easily infatuated with the small things and has a need to see everything.

Much like Atsushi, she believed that she was worth very little and that she deserved to die for her crimes. Although unlike Atsushi, once she joins the Armed Detective Company she does not dwell on her mistakes. She is strong-willed and even states that she wants to prove to Akutagawa that she is more than just a killer.

It is mentioned in chapter 18 by Kouyou that before being an orphan, she supposedly lived happily with her parents, But ultimately it was the Demon itself who killed them. Since then Kouyou took her and served under the Mafia as her protegee.

After Joining Armed Detective Company Fukuzawa fixes the problem which enables her to control her ability and use it by herself.

(Sources: Easy Going Scans, Bungou Stray Dogs Wikia)