antagonist
an-tag-o-nist
noun
A character or force opposing the protagonist in a narrative.
"The villain was the story's main antagonist."
noun
A muscle whose action opposes that of another muscle.
"The triceps is an antagonist to the biceps."
noun
A substance that inhibits or counteracts a physiological effect.
"Naloxone acts as an opioid receptor antagonist."
noun
A person who actively opposes or is hostile to another.
"He became a fierce antagonist to the new policy."