Moushumi is perhaps Gogol's primary antagonist. While the other breakups in the novel are mutual, Moushumi betrays Gogol. She does this not only by cheating on him, but also by destroying the illusion that he has reconciled the Indian and American elements within himself.
Still, she doesn't bear all the blame. The novel takes pains to tell her side of the story, and in a way, she is the protagonist of her very own chapter. It's hard to hate Moushumi, which makes us a bit reluctant to call her antagonist. Still, she's the closest thing we have.