This would seem pretty straightforward, but if we look at “Young Goodman Brown” as a dream, it has a twist. First, put simply, Young Goodman Brown is the protagonist. You could also say that Faith is a second protagonist, especially since her name is a pun on Young Goodman Brown's own faith. Thus, his faith, a part of his own mind/soul, must necessarily be a part of himself. That pun aside, Brown himself is the protagonist. The devil is the antagonist. These characters could be real or part of Brown's dream, part of his own consciousness. It would seem that this is a dream vision. That being said, Brown still believes it fully because the next day, he has clearly lost faith in others. “A stern, a sad, a darkly meditative, a distrustful, if not a desperate man, did he become, from the night of that fearful dream.”
And here's the twist. In this case of this being a dream, you could say that Brown is the protagonist and the antagonist. This is a battle in his own mind. Once he becomes aware of, or suspects, that his fellow church-goers are sinners, he loses his faith in them and then in himself. He begins to believe that “Evil is the nature of mankind.” Brown struggles with his own faith and his own conscience.
Brown's inability to accept the potential for people to be bad and good, faithful but flawed, is Hawthorne's indictment of the extreme black and white philosophy of Puritan religious beliefs.