Antagonist
The force opposing the protagonist — a character, institution, nature or inner flaw. A good antagonist is right from their own point of view.
The antagonist is the story's opposing force: what stands between the protagonist and their goal. Often a character, but not necessarily — an institution (the law in Les Misérables), a milieu, nature, or the hero's own flaw can hold the role.
The golden rule of great antagonists: they are the heroes of their own story. Javert isn't evil — he serves the law with absolute integrity; that's exactly what makes him terrifying and tragic. An antagonist who wants "evil" threatens nothing; an antagonist who wants a good incompatible with the hero's structures the entire story.
Example
Javert in Les Misérables: integrity itself, turned into a crushing force.