Nouns

(n)protagonist, agonistthe principal character in a work of fiction
(n)agonistsomeone involved in a contest or battle (as in an agon)
(n)agonista muscle that contracts while another relaxes“when bending the elbow the biceps are the agonist”
(n)agonist(biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction