Onegai☆Teacher

Please Teacher: Kusanagi Kei, a high-school student living with his aunt and uncle, has an encounter with a female alien. This alien is revealed to be a new teacher at his school. Later, he is forced to marry this alien to preserve her secrets. From there, various romantically-inclined problems crop up repeatedly. Please Twins: Maiku Kamishiro's past has always been somewhat of a mystery to him. The only clue he has to who his family might be is an old photograph showing two young children, a boy and a girl, playing in a small pool outside of a blue house. In an attempt to find his family, he moves to this blue house and begins working as a programmer. He's living a comfortable life until the day two very different girls show up at his door, both in possession of the same photograph, and both claiming to be his twin. With no way to prove who is a relative and who is a stranger, Maiku allows both girls to move in with him until they know for sure. (Source: Anime News Network) Please note that "Please Teacher!" and "Please Twins!" are only spin-offs of each other and are not related in any other way.