Kamikakushi no Eden

Kuroto Hinata is searching for his sister, who "once existed" but is now forgotten by everyone around him. He receives a letter from the beautiful transfer student, Shirai Shizuku, stating, "If you want to know about the whereabouts of your sister, come to the library after school." When Kuroto goes to the library, he finds Shirai inexplicably half-naked and waiting for him.