Koike Aya is a 9 year old girl who Ida crosses paths with on a case in volume 7 of Jiraishin. She appears to be a sociopath and very mature for her age. She also shows no remorse for those who have died not even her friends. As a result of her mischief Ida is caught in the middle of what she believes to be a game between two forces called Thrill and Real. This results in Ida having to go up against a killer that she drove over the edge who now intends to kill her.