I have been reading a lot of blog posts recently about developing your antagonist – what characteristics he should possess, how clearly defined his motivations need to be, etc . . . They all indicate that a strong, well-developed antagonist is essential to your overall ARC. Problem is they all assume that the antagonist has to be an actual person. That is not always the case.
The antagonist can be the society, the environment, and in some cases even the protagonist himself. In Jack London’s “To Build A Fire” The antagonist is the bitter cold winter. In Cormac McCarthy’s dystopian “The Road” the antagonist is the post-apocalyptic environment.
Can the protagonist and antagonist be the same person? Sure can, but that one’s hell of a hard book to write. Look at Herman Melville’s “Moby Dick.” Many say that the antagonist is the whale. Equal numbers have challenged that the antagonist is actually the mc’s obsession with the whale itself.
The antagonist is whatever opposes your mc in a fight, conflict, or battle of wills. The simplest and most common are villains, that one evil character your mc must overcome. Although the development of this “character” is vital, it is important to understand that the antagonist doesn't always have to be another human character. Person, place, or thing, so long as it is well developed you have accomplished your goal.