Koike Aya is a nine-year-old girl whom Ida encounters during a case in volume 7 of Jiraishin. She exhibits traits of a sociopath and displays an unusually mature demeanor for her age. Aya shows no remorse for the deaths of others, including her friends. Her mischief leads Ida into what she perceives as a game between two forces named 'Thrill' and 'Real,' resulting in Ida facing off against a killer who has been driven to the brink because of her actions.
In the end, Ida outsmarts the killer by making him believe he can escape with Aya. Aya panics, fearing that Ida has abandoned her to die. Ida ambushes the killer and neutralizes him with a single shot. Believing that the blood-covered Aya has learned her lesson, they part ways.
Aya reappears in volume 15, three years later, showing no remorse for the four deaths caused by her actions in the past. She assists Ida on another case, seemingly unchanged by her previous experiences.
