agonist
     n 1: the principal character in a work of fiction [syn: {protagonist}]
     2: someone involved in a contest or battle (as in an agon)
        [ant: {adversary}]
     3: a muscle that contracts while another relaxes; "when bending
        the elbow the biceps are the agonist"
     4: (biochemistry) a drug that can combine with a receptor on a
        cell to produce a physiological reaction
Source: WordNet® 2.0