She was Kouta's younger sister between the ages of 8-10 years old. She was protective of her brother and was jealous of other girls' attentions to him. Kohta cares about her very much, but repressed much of his memory of her after Lucy murdered her on a train along with Kohta's father. Kohta's repression of the events leads him to believe she died of a long sickness of some kind

(Source:Wikipedia)