Definition of nemeses:
(n) :
An enemy, especially an archenemy.
(n) :
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.
(n) :
(usually in the singular, formal) A punishment or defeat that is deserved and cannot be avoided; (an instance of) retribution.
(n) :
The principle of retributive justice.
(n) :
The personification of the "fatal flaw" of a dramatic hero in the style of Greek tragedy.
(n) :
Polar opposite, especially of a literary character.