Kono Aozora ni Yakusoku wo: Melody of the Sun and Sea

One morning, Wataru Hoshino found a sleeping girl - in nothing but her underwear - in his room. When she woke up, she quickly escaped by jumping out of the window, but not without punching Wataru in the face first. This girl, Rinna Sawaki, ends up being a transfer student of Wataru's school, as well