Okay so I'll be posting a CS for a human and alien character, but I had an idea regarding aliens. Basically, in this galaxy humans are the dominant race so the reason aliens could be so rare is that each race comes from a different galaxy. Galaxies are really far apart so any alien that finds itself in the human-dominated galaxy would be considered just that: an alien.

Just a thought. Also, aliens would probably need a good excuse for being in the human-dominated galaxy, it wouldn't exactly just be a vacation.