Amethyst

Amethyst is the only antagonist ala Swiper the Fox or the Kooky Kid. She debuted in The Truth.

Trivia
Unlike Swiper (Oh, man!) and the Kooky Kid (That's Kooky!), Amethyst can say so much things, for examplee in The Airport, "JFK, Honolulu," and in The Package, "I love the health club."