noun
A person or entity that stands in opposition, often in a conflict, competition, or narrative. Slightly formal or literary.
彼は長年の敵対者を打ち負かした。
He defeated his long-time opponent.
物語の敵対者は、主人公の幼なじみだった。
The antagonist of the story was the protagonist's childhood friend.
敵 is a more common and broader word for 'enemy', while 敵対者 emphasizes the role of an active opponent or antagonist.
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.