Kanae

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.

[Spoiler] Kouta cares about her very much, but repressed much of his memory of her after Lucy murdered her on a train along with Kouta's father. Kouta's repression of the events leads him to believe she died of a long sickness of some kind. [/Spoiler]