Kamikakushi no Eden

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

(Source: Kadokawa, translated)