noun (n)

• the principal character in a work of fiction(noun.person)

  Synonym:
  protagonist
  

  source: wordnet30
• someone involved in a contest or battle (as in an agon)(noun.person)

  Antonym:
  adversary, antagonist, opponent, opposer, resister
  

  source: wordnet30
• a muscle that contracts while another relaxes(noun.body)

  Example:
  When bending the elbow the biceps are the agonist.
  

  source: wordnet30
• (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction(noun.artifact)

  source: wordnet30
• One who contends for the prize in public games.(noun)

  source: webster1913