agonist

noun

1.
the principal character in a work of fiction
protagonist
Derived from:
agony
2.
someone involved in a contest or battle (as in an agon)
Antonym:
antagonist
Derived from:
agonistic agony
3.
a muscle that contracts while another relaxes
4.
(biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction