A character in conflict with the protagonist.

A muscle that contracts to create movement.

A drug that activates a receptor in biology.

**Pharmacological agonist**: Substances activating specific receptors in biology.

**Theatrical protagonist**: Main character or lead role in drama.

**Sports competitor**: Athlete engaged directly against an opponent.

**Muscle contraction agent**: Muscle that contracts during movement action.

**Conflict participant**: Individual involved in a struggle or contest.