Antagonist

The antagonist is whatever opposes the protagonist's goals. While often a villain, the antagonist can also be a societal system, nature, or even the protagonist's own flaws. The antagonist's role is to create conflict that forces the protagonist to grow, adapt, or ultimately fail. Without a compelling antagonist, the protagonist's journey lacks tension.

In Moby-Dick, the white whale functions as an antagonist that is also a symbol. In The Shawshank Redemption, the warden is the direct antagonist, but the prison system itself serves as a broader antagonistic force. In Hamlet, Claudius is the external antagonist, but Hamlet's own indecision is arguably the more powerful opposing force.

The best antagonists believe they are the hero of their own story. A villain who is evil for evil's sake is far less interesting than one whose motivations, while opposed to the protagonist's, make internal sense. Thanos in Avengers: Infinity War succeeds as an antagonist because his logic, however twisted, is internally consistent.