DearS 

In this raunchy sci-fi anime, an alien spacecraft has crash-landed in Tokyo Bay and the extraterrestrials start going to high school with human teens. When skeptical student Takeya finds one of these lost aliens, called 'DearS,' he gets over his initial doubts and takes her home to live with him--but now she considers him her "master!" Add a cat-girl and an over-sexed teacher, and Takeya's life is beginning to become a little more than he can handle.