It could be a powerful demon lord from a dark anime universe. This demon lord might seek to conquer all the anime universes in the 'anime war infinite' setting. He could have an army of dark creatures and powerful magic at his disposal, making him a formidable foe.
Maybe the main antagonist is a jealous rival. This rival is envious of her abilities and status, so she uses a forbidden magic to turn her into a card. The rival then tries to take her place in the story, but her friends start to suspect something is wrong and begin to investigate.
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.