Himemiya Nanahoshi

He may look like an ordinary guy, but he's actually a space alien.
The power source to his spaceship went missing and while looking for it he mistook the stone the protagonist holds for it and attacked her, but he ends up living at her house somehow and because he doesn't have a last name he takes hers to fit in.

A serious and straight-laced guy, but because he doesn't have common sense about Earth things he does weird things with a serious intent.