antagonist
/ænˈtæɡənɪst/
An opponent or enemy.
One who antagonizes or stirs.
A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.
2001: The calcium antagonists represent one of the top ten classes of prescription drugs in terms of commercial value, with worldwide sales of nearly $10 billion in 1999. — Leslie Iversen, Drugs: A Very Short Introduction (Oxford 2001, p. 41)
(authorship) The main character or force opposing the protagonist in a literary work or drama.
A muscle that acts in opposition to another.
A flexor, which bends a part, is the antagonist of an extensor, which extends it.