* Philosopher's Stone: Voldemort is the main villain, but he's not physically present. His presence is felt through his attempts to steal the stone and regain his body.
* Chamber of Secrets: Voldemort's diary acts as a vessel for his spirit and is the main antagonist. He's not physically present until the very end.
* Prisoner of Azkaban: Sirius Black, wrongly accused of murdering 12 Muggles and betraying the Potters, is the initial antagonist. However, his motivations are more complex and he's ultimately revealed as a good character.
* Goblet of Fire: Voldemort returns to power, becoming the primary antagonist. However, he's not the only villain; the Triwizard Tournament itself presents a danger to Harry and others.
* Order of the Phoenix: Dolores Umbridge is the primary antagonist for a large portion of the book, as she establishes oppressive policies and persecutes Harry and his friends. Voldemort's influence is also present, but he doesn't physically appear until the end.
* Half-Blood Prince: Voldemort is the main antagonist, but Draco Malfoy's attempts to fulfill a Dark Mark task and his internal conflict also play a role.
* Deathly Hallows: Voldemort is the main antagonist throughout, culminating in the final battle.
Therefore, while Voldemort is the main antagonist throughout the series, other characters like Sirius Black, Dolores Umbridge, and even the Triwizard Tournament itself contribute to the conflict in individual books.