 If one was to watch closely you can tell it isn't her for a number of reasons.In the final episode the traitor was revealed to be Jen, although she was being controlled by a demon (that possessed her on the first episode, when she disappeared while with Byron) who would take her soul for themself instead of Killian having it.In the end when they defeated Killian with the demon from Karl's curse, the Rabbit, the whole of Room 21 was stuck in Killian's world even though he was defeated.