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

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