At first, Beatty is simply Montag's boss, the Captain. In this relationship, Beatty brings out obedience in Montag. But shortly after the novel begins, and following Montag's encounters with Clarisse McClellan, Beatty comes to represent an authority: one which Montag fears and eventually endeavors to oppose.
After Clarisse disappears, Montag is playing cards with Beatty and the other firemen. He has now become a thinking man and he is experiencing feelings he never had before. At this moment he feels guilty - basically for thinking by himself:
Beatty was looking at him as if he were a museum statue. At any moment, Beatty might rise and walk about him, touching, exploring his guilt and self-consciousness. Guilt? What guilt was that?
Beatty has some knowledge of books and history. He is the one to tell Montag that "Master Ridley" (quoted by the woman who burned with her books) was a man condemned and burnt for heresy in 1555. When Beatty visits Montag after this fire, he acknowledges that some firemen get curious about books but that it should not last more than 24 hours. Beatty is trying to persuade Montag that some firemen do go through a phase such as this and that it is just a phase. Beatty tries to convince Montag that equality means that all people are made equal, are all the same; not that they have equal opportunity to be who they want and/or read what they want. Despite this strategy, Montag just becomes more curious and more suspicious of Beatty's reasoning.
In the end, Beatty becomes Montag's enemy. Beatty is angry that Montag would ruin his (Montag's) life in this way. Although Beatty does have some knowledge of books and poetry, he is intransigent; he is sticking to his belief that burning books makes the world a better place. Beatty is an authority figure that stands in the way of Montag's new life: of thinking for himself, of being free to read and imagine a new way of living.