An archenemy, archfoe, archvillain, or nemesis (sometimes spelled arch-enemy, arch-foe, or arch-villain) is the principal enemy of someone or something. In fiction, it is a character who is the hero/protagonist's worst enemy.
Etymology
The word 'archenemy' or arch-enemy originated around the mid-16th century, from the words arch- (from Greek "arkhos" meaning "most important") and enemy.
References
Archenemy Wikipedia(Text) CC BY-SA