This is book number 1 in the Studio Apartment, Good Lighting, Angel Included series.
Shintaro Tokumitsu is a high schooler living all alone, but things take an unexpected turn when a girl named Towa shows up on his balcony! Not only is she incredibly pure and sweet, but there’s something different about her—something...divine. Just who is Shintaro’s new roommate, and what adorable high jinks lie in store?!