Previously, I went over what a protagonist actually is (and what it isn’t), and I gave some defining attributes to help you determine if your character is truly the protagonist. So it only seems natural that I do a follow-up on something similar: what an antagonist truly is.
Of course, though, not all antagonists are characters.Some stories may not even have an antagonistic person. The antagonist could be nature, like a hurricane or a disease. It could be technology, like a computer virus. It could be a religion. With that said, though, many stories do have an antagonistic character.