Who is the antagonist in the novel Harry Potter and the Prisoner of Azkaban?

The true antagonist in this novel is, as usual, Voldemort. However, for a long time it appears to be Sirius Black. Many around Harry are sure that Black, who has escaped from Azkaban prison, is a firm supporter of Voldemort. At first, the evidence seems to support this idea. For example, Arthur Weasley, a person Harry trusts, tells him that Sirius Black plans to murder Harry and that Black has a track record as a murderer. Harry also overhears teachers trading what turn out to be false stories about Black, such as that he betrayed Harry's parents, who had been his friends, to the treacherous Voldemort, and that he murdered Muggles.
In fact, it turns out that Sirius Black is Harry's closest living relative and wants very much to protect him from Voldemort. Voldemort has taken control of Akzaban prison and, therefore, has dementors under his control, which he plans to use to destroy Harry.