Antagonist

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.