A nemesis is the principal enemy of someone or something. Does it really represent an enemy? In vernacular English, archenemy and nemesis are the two from the four above that are often used.

Why is there no single English word to describe the single most important enemy in which case you need to combine

• two words:
• Primary Adversary
• Main Opponent Sole Archenemy or

Sole Nemesis Note to OP: This name still applies as to enemy.