Betters is the romantic pairing of Bebe Stevens and Butters Stotch.
Butters was one of the many boys who though Bebe was really cool in "Bebe's Boobs Destroy Society". When Wendy exclaims that the boys are so stupid, Butters replies "Stupider than Bebe!"
There are quite a few fanfictions regarding this pairing. There are also many fanarts regarding it.