You are Ms. Servant.

Hitoyoshi, a high school kid who lives alone, hears a knock on his door. He opens it to find a strange girl offering to be his maid. She has beautiful black hair, graceful manners, and…a dark past as an assassin. But despite her deadly background and lack of household skills, Hitoyoshi takes her in. As she adjusts to her new life, she begins to experience emotions she’s never felt before.