nemesis

noun

1. An enemy, especially an archenemy.

   “Batman is in constant conflict with his nemesis, The Joker.”

   “The roofer stayed in touch, committed to the downfall of his nemesis, Mikey. One week after his initial phone call, we were on our way to raid Mikey's trite home armed with a federal search warrant. There were six of us.”

   “They were different in many ways but what was it about opposites attracting... […] She was his opposite, but not his nemesis, and they worked well together. She hoped he liked working with her.”

   “Jack took many hits to his body from his nemesis, but to his surprise he barely felt it. Jack's nemesis looked at him with anger in his eyes, clinching his fist to throw the next blow and without hesitation he threw another punch[…]”

   “Along the way, they meet Master Wong, who teaches them kung fu and imparts nuggets of Chinese philosophy, knowledge and training that they eventually use to defeat their nemesis and save their village.”

2. A person or character who specifically brings about the downfall of another person or character, as an agent of that character's fate or destiny, especially within a narrative.
3. The personification of the "fatal flaw" of a dramatic hero in the style of Greek tragedy.
4. Polar opposite, especially of a literary character.

   ““And then, while he's devouring you with his stare, thinking about how much he wants to lick you up, along comes his antithesis, his polar opposite, his nemesis for want of a better word—” “Nemesis?” I interrupted, eyebrows journeying up my forehead. “—who swoops you off your feet and away from him,” Heather continued, ignoring my incredulous expression.”

5. The principle of retributive justice.
6. (formal, singular, usually)A punishment or defeat that is deserved and cannot be avoided; (an instance of) retribution.

   “Even Nathan's close associates, who are not yet ready to declare themselves his enemies, accept that his engineered downfall will be his nemesis for harbouring ambitions beyond his status, and do nothing to alert him to the situation[…]”

name

1. (Greek)The goddess of divine retribution.
2. 128 Nemesis, a main belt asteroid.
3. A hypothetical star postulated to be orbiting the Sun to explain a perceived cycle of mass extinctions in the geological record.