All 3 Uses

antagonist
in
This Side of Paradise
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As an actress even in the fullest flush of her own conscious magnetism gets a deep impression of most of the people in the front row, so Isabelle sized up her antagonist.†
Chpt 1.2 *

antagonist = someone who offers opposition or is hostile
Your last, that dissertation on architecture, was perfectly awful— so "highbrow" that I picture you living in an intellectual and emotional vacuum; and beware of trying to classify people too definitely into types; you will find that all through their youth they will persist annoyingly in jumping from class to class, and by pasting a supercilious label on every one you meet you are merely packing a Jack-in-the-box that will spring up and leer at you when you begin to come into really antagonistic contact with the world.†
Chpt 1.3

antagonistic = with opposition or hostility
"Kids—child's play!" scoffed his antagonist.†
Chpt 2.5

antagonist = someone who offers opposition or is hostile

Definitions:

(1) (antagonist as in: the antagonist in the film)
someone who offers opposition or is hostile
(2) (meaning too rare to warrant focus)
Specialized Definitions of antagonist:
- literature or storytelling: the most important character who opposes the main character or protagonist
- physiology: a muscle that relaxes when another muscle contracts
- chemistry and biology: a substance that interferes with the effect of another substance