Antagonist:
1. One who opposes and contends against another; an adversary.
2. The principal character in opposition to the protagonist or hero of a narrative or drama.

Thesaurus Synonyms:
* adversary
* agonist
* dueler
* duelist
* dueller
* duellist
* enemy
* foe
* foeman
* opponent
* opposer
* opposition
* resister
* withstander