Someone who only cares for themselves and thinks the world revolves around them.
Therefore payback is talking behind your back to friends.
David - "Sweet, thanks buddy!"
*Later that day*
Brad - "Hey Dave! Check out my WICKED shirt! SOrry, couldnt get you one, but you suck"
David - ":( your a bad friend bradley...."
Usually when out the prettier friend will be told how good looking she is all the time and admired by men and the uglier friend is ignored.
The prettier friend everyone will think is a model and all the guys hit on and the uglier friend gets ignored.
Then years later the ugly friend realizes what a failure her life is and how she will never have the brains or beauty to be successful or rich in life and she comes on the internet to hate and make up lies about the girl.