antagonist

Short definition from Wiktionary

Noun

1. 1
   An opponent or enemy.
2. 2
   One who antagonizes or stirs.
3. 3
   A chemical that binds to a receptor but does not produce a physiological response, blocking the action of other agonist chemicals, such as endogenous chemical messengers.
4. 4
   The main character or force opposing the protagonist in a literary work or drama.
5. 5
   A muscle that acts in opposition to another.