2. The principal character in opposition to the protagonist of a narrative or drama.
3. Something opposing or resisting the action of another; certain structures, agents, diseases, or physiologic processes that tend to neutralize or impede the action or effect of others.
4. In biochemistry, a chemical substance that interferes with the physiological action of another; especially, by combining with and blocking its nerve receptor.
2. One of the main characters in a book, play, or movie whose values or behaviors are in conflict with those of the primary character: The objective of the antagonist in the story was to destroy the protagonist in battle.
"An "antagonist" is really someone who is opposed to another person; an "opponent".
At a recent production of story of the prince and the poor girl, the character of the stepsister who was the antagonist was exaggerated and made the audience laugh; however, the poor girl who was the protagonist in the story was portrayed as gentle and sweet.