agonist

from WordNet (r) 3.0 (2006)

agonist
    n 1: the principal character in a work of fiction [syn:
         {protagonist}, {agonist}]
    2: someone involved in a contest or battle (as in an agon) [ant:
       {adversary}, {antagonist}, {opponent}, {opposer}, {resister}]
    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

from The Collaborative International Dictionary of English v.0.48

Agonist \Ag"o*nist\, n. [Gr. ?.]
   One who contends for the prize in public games. [R.]
   [1913 Webster] Agonistic