The antagonist of a story is the 'villian' or 'bad guy' in the story. In East of Eden the antagonist is Cathy.
Cathy Ames is described by the narrator as a monster. She is consistently evil in her thoughts and actions, manipulating others for her own ends without a trace of conscience. Cold and callous, she seems to be without a single decent feeling. Cathy is the only daughter of a respectable family in Massachusetts. As a young girl she is different from the other children; she is a nonconformist and a liar. At the age of ten she gets two boys punished for indulging in sex play with her, which she initiated; at high school she drives her Latin teacher to suicide. At sixteen she murders her parents by burning down the family home.