I think this is right:
If your in a club and u see a really hot girl, then as u get near, you see she has a really ugly friend who puts you off so you walk away - thats the butters girl
oh man, i was this close, then i saw the butters girl
The Butters Girl is always the friend of the fittest girl in the club.
She becomes affective at the point of chatting up the fit girl, where she quickly swifts her off to the loo or something, just because she does not get laid often and does not want to be left alone.
See also cockblocker
Ah man, I was just about to put that girl away till the butters girl came along.