"agonist" in the noun sense
1. protagonist, agonist
the principal character in a work of fiction
someone involved in a contest or battle (as in an agon)
a muscle that contracts while another relaxes
"when bending the elbow the biceps are the agonist"
biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction
Source: WordNet® (An amazing lexical database of English)
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