The antagonist is a major part of any story. Sometimes they are even more important to the progression of the plot than the protagonist, and in some cases it may be hard to even distinguish the two.
What makes the antagonist the antagonist?
It’s not because they are evil though some antagonists are. It’s not because of their dark pasts because protagonists have those to (such as Batman). The antagonist is only the antagonist of a story because their motivations and goals are in a completely different direction than those of the protagonist.
It’s not because of morals, or actions, or anything like that. The antagonist of a story may not even be noticeable in the story. They may hide in the background of scenes containing the protagonist. They may even be protagonists in their own right because if they become the protagonist of the story its the current protagonist who become the antagonist.