If you consider the way that Faber and Beatty seem to engage in a furious battle for Montag's soul in the middle section of the book, it becomes clear that the central protagonist is Beatty himself. Of course, he does stand for the entire system of repression and censorship that dominates Montag's world, but he is the main opponent that Montag faces. Note how he presents his views to Montag, in opposition to what Faber says:
"Well," said Beatty, "the crisis is past and all is well, the sheep returns to the fold. We're all sheep who have strayed at times. Truth is truth, to the end of reckoning, we've cried. They are never alone that are accompaneid with noble thoughts, we've shouted to ourselves."
Of course, ironically, Beatty used quotations from literature to support his argument and also to refute the possible arguments of Montag in response. It is Beatty that stands for the system of repression that Montag rages against, and it is Beatty that Montag burns in response, indicating his position as the antagonist of the novel.