Antagonist

The force organized against the protagonist's goal — a person, an institution, or the flaw within.

The antagonist is whatever pushes back: a rival, a system, a storm, an addiction. It doesn't require malice — it requires opposition, ideally the kind purpose-built to press on the protagonist's specific weakness.

A useful check: your antagonist should be the hero of their own version of the story. Opposition with its own logic escalates; a cackling obstacle just repeats.