antagonist

noun

character of a work actively opposing the protagonist
muscle that moves part of the body in the opposite direction from another muscle
class of pharmacological agents, which reduces or diminishes the effect of an agonist on a receptor

Wiktionary

Pronunciation: /ænˈtæɡənɪst/ / [ɛə̯nˈtæɡənɪst]

noun

Etymology: From Latin antagonista, from Ancient Greek ἀνταγωνιστής (antagōnistḗs, “opponent”) (ἀντί (antí, “against”) + ἀγωνιστής (agōnistḗs, “a combatant, pleader, actor”)), from ἀνταγωνίζομαι (antagōnízomai, “to antagonize”).

An opponent or enemy.
“our antagonists in these controversies”
“Antagonist of Heav’ns Almightie King”
One who antagonizes or stirs.
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.
“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)”
The main character or force opposing the protagonist in a literary work or drama.
“So God forbid I'm seen just as an average human being / I mean, imagine if antagonists lacked any evil scheme”
A muscle that acts in opposition to another.
“A flexor, which bends a part, is the antagonist of an extensor, which extends it.”