The big twist is that they're ALL traitors.
Inquisitor: All right, in the wake of the discovery that Morrigan was actually Flemeth in disguise, I have to ask: who in this room is secretly working for someone else?
Vivienne: My loyalty has always been to Empress Celene, darling.
Sera: And I'm working for Gaspard de Chalons. Hey, he pays well.
Cassandra: I regret to inform you that I am secretly an agent of the Red Templars.
Cullen: And I was planning to capture you for the regular Templars.
Solas: I'm possessed by Imshael, but I'm technically working for myself.
Dorian: I'm with the Venatori, old boy. "He shall reign" and all that.
Iron Bull: Since we're airing out our dirty laundry, I never left the Qun.
Varric: Hawke sent me to keep an eye on you and take you out if you turned evil, Quizzy.
Leliana: I confess that I am also Flemeth. Bodies are such limiting things.
Cole: I'm not working for anyone, but I was thinking about killing you anyway. Would you look into my eyes for a moment?
Feathers: My allegiance is to the Wardens, young one. Also, Corypheus.
Inquisitor: Right, I've heard enough. Is there ANYONE in this organization loyal to me?
(A ghastly whinny echoes through Skyhold)
Inquisitor: Thank you, Bog Unicorn.