Nemesis

Nemesis is the Greek goddess of revenge.

History
Nemesis is the daughter of Oceanus, a water god. She appears chiefly as the avenger of crime and the punisher of hubris.

In the Series
She was mother to Ethan Nakamura, and sided with Kronos when faced with choosing between siding with the Titans or siding with the Olympians.

Personality
Nemesis is a cold hearted goddess, and demands signs of loyalty by maiming her children, such as blinding Ethan in one eye. Percy comments that he wouldn't want a mother like her.

Other
Nemesis is now often used as a term to describe one's worst enemy, normally someone or something that is the exact opposite of oneself but is also somehow similar. For example, Luke is frequently described as the nemesis of Percy.