Yes, AGONIST is a Scrabble word!

Letter values in Scrabble US: A 1, G 2, O 1, N 1, I 1, S 1, T 1.

Is it a valid word?

AGONIST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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