Helena Bonham Carter as Marla Singer / Fight Club / Fun fact: the flower ring that Helena is wearing throughout the movie is a box ring. According to the actress in the DVD commentary it is supposed to be Marla's "coke [cocaine] ring".
Welcome to Fight Club. The first rule of Fight Club is: you do not talk about Fight Club. The second rule of Fight Club is: you DO NOT talk about Fight Club! Third rule of Fight Club: someone yells "stop!", goes limp, taps out, the fight is over.