This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. One who opposes and contends against another; an adversary.

n. The principal character in opposition to the protagonist or hero of a narrative or drama.

n. Physiology A muscle that counteracts the action of another muscle, the agonist.

n. Biochemistry A chemical substance that interferes with the physiological action of another, especially by combining with and blocking its nerve receptor.

n. An opponent or enemy.

n. One who antagonizes or stirs.

n. A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.

n. The main character or force opposing the protagonist in a literary work or drama.

n. One who contends with another, especially in combat; an adversary; an opponent.

n. A muscle which acts in opposition to another.

n. A medicine which opposes the action of another medicine or of a poison when absorbed into the blood or tissues.

adj. Antagonistic; opposing; counteracting.

n. One who contends with another in combat or in argument; an opponent; a competitor; an adversary.

n. In anatomy, a muscle which acts in opposition to another: as, a flexor, which bends a part, is the antagonist of an extensor, which extends it.

Counteracting; opposing; combating: as, antagonist forces; an antagonist muscle.

n. a drug that neutralizes or counteracts the effects of another drug

n. someone who offers opposition

n. a muscle that relaxes while another contracts

"About halfway through (when the main antagonist is killed) every single page was a stop-or-keep-going decision."

protagonist   

antagonistic muscle   

enemy    opponent    foe    competitor    adversary    antagonistic    opposing    counteracting   

protagonist   

adversary    opponent    foe    competitor    assailant    receptors    rival    ally    warrior    fighter