agonist

Noun

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

• a muscle that contracts while another relaxes
  • when bending the elbow the biceps are the agonist
    

• someone involved in a contest or battle (as in an agon)

• the principal character in a work of fiction

Related words

• Antonyms: 
  adversary , antagonist , opponent , opposer , resister