costello
someone who is very immature and sarcastic at times, but ultimately funny in being so. This person also has sadistic ways of going about things, but can be very serious about certain things. Costellos always push people to do their best. They are also very soft on the inside, and may disguise liking you in forms of teasing and sarcastic comments. All in all, Costello's are very beneficial to have in your life if you know their rules and play by them.
My swim coach is such a costello, but he's the best coach I've ever had.