A person or entity that stands in opposition, often in a conflict, competition, or narrative. Slightly formal or literary.
He defeated his long-time opponent.
The antagonist of the story was the protagonist's childhood friend.
Compound of 敵対 (てきたい, 'hostility, antagonism') and 者 (しゃ, 'person'). The exact historical derivation is uncertain, but the word follows a standard Sino-Japanese pattern for forming agent nouns.