Yumiko is an elementary school student. She is a genius with IQ of 300. But she has a dark secret. She kills without a reason. She is a genuine psychopath and she is after Arayas life. She reffers to her killing as an experiment.
Later it is revealed she feels no pain. After assaulting Arayas class and pocking out Gotous eye she was detended in jail but later escapes leaving a bloodbath in the detention facility. She accepts Araya as her only opponent.