It depends on the specific story. Different stories have different antagonists. Sometimes it could be a villain with evil intentions, and other times it might be a character with opposing goals to the protagonist.
An antagonist is basically a character or a force that opposes the main character or protagonist. They create conflicts and obstacles for the hero to overcome.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The villain can be an antagonist in story. For example, in the 'Harry Potter' series, Voldemort is a clear antagonist. He tries to harm the main characters and has goals that are opposed to the heroes.
The Joker from Batman comics and movies is also a great antagonist. He is chaotic and unpredictable. His actions are not driven by a desire for power or money in the traditional sense but rather by a love for chaos and anarchy. He constantly challenges Batman both physically and mentally, making him an iconic villain.