Based on the erotic game by Terios.

A magical creature went to the normal world to retrive some seeds. He lost his power and fainted. A girl found him and took him home. Well, this creature became a man after gaining back his power, which surprised the girl some. An incident happened and she became a magical fighter with his kiss. Things start to get complicated as they try to get the seeds and with most people not being what they seem.

(Source: AniDB)