Kamikakushi no Rakuen

Hinata Kurodo is searching for his sister, who "once existed" but who disappeared, and now no one else remembers her. Kurodo receives a letter from a transfer student, a beautiful girl named Shimotsuki Shirai, saying, "If you want to know where your sister is, come to the library after school." Kurodo goes to the library, where he finds a half-naked Shirai eagerly waiting for him…
(Source: Comic Vamp, translated)