Enemy

—a person who feels hatred for, fosters harmful designs against, or engages in antagonistic activities against another; an adversary or opponent.
—an armed foe.
—a hostile nation or state.
—a citizen of such a state [Dictionary.com].
An enemy is a person who actively opposes someone or something. The Latin word inimicus, meaning "hostile, unfriendly," is the root of enemy, and it comes from the prefix in-, or "not," and amicus, "friend:" an enemy is "not a friend." When two armies fight, they think of the opposing army as the enemy [Vocabulary.com].
A person who hates or opposes another person and tries to harm them or stop them from doing something [Cambridge Dictionary].

Sources:

Enemy. Dictionary.com. Retrieved from: https://www.dictionary.com/browse/enemy

Enemy. Vocabulary.com. Retrieved from: https://www.vocabulary.com/dictionary/enemy

Enemy. Cambridge Dictionary. Retrieved from: https://dictionary.cambridge.org/dictionary/english/enemy