Denpa Onna to Seishun Otoko

A high school boy named Niwa Makoto lives with his aunt's family while his parents are away on business. There, he meets his cousin Touwa Erio, a girl of the same age who claims to be an alien. Erio carries a futon mattress around her upper body and eats pizza regularly. She had been missing for six months before being discovered floating in the sea. Erio has no memory of that time but believes it was caused by aliens. Since then, she has been wandering the neighborhood wrapped in her futon.