'agonist' definitions:

The principal character in a work of fiction [syn: protagonist, agonist]

Someone involved in a contest or battle (as in an agon) [ant: adversary, antagonist, opponent, opposer, resister]

A muscle that contracts while another relaxes; "when bending the elbow the biceps are the agonist"

(biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction