Tenchi Muyo! Ryo-Ohki

Tenchi Masaki was a normal 17-year old boy - until the day he accidentally released the space pirate Ryoko. She had been sealed in a cave 700 years ago, due to the locals' suspicions that she was a demon. In a series of events, many more alien girls show up at the Masaki household. Tenchi learns much of his heritage he never knew about while dealing with the alien girls - each of whom has some sort of romantic interest in him.