agonist (Noun) — The principal character in a work of fiction.
agonist (Noun) — Someone involved in a contest or battle (as in an agon).
agonist (Noun) — A muscle that contracts while another relaxes. ex. "when bending the elbow the biceps are the agonist"
agonist (Noun) — (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction.