antagonist

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.