Nemesis

Nemesis (Νέμεσις) is the Greek goddess of revenge and balance.

History
In some myths Nemesis is the daughter of Oceanus, the sea Titan, yet in others she appears as the daughter of Nyx (personification of night), created from Chaos, the mass of energy at the begininng of time. She appears chiefly as the avenger of crime and the punisher of hubris. Her name in Greek means "to give what is due" therefore giving her the nickname, Justice.

Percy Jackson and the Olympians
Although she was not seen 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. She gets her own cabin after The Last Olympian. She doesn't play a major role in the series, besides being the mother of Ethan Nakamura. At some point she made an exchange with her son, he would give up his eye in exchange for bringing minor gods respect and power. Ethan gave more than his eye, but the minor gods were given their own cabins and general amnesty around Olympus.

Nemesis is the symbol of balance.

Nemesis may of been one of the minor gods gathering for the Nine Muses concert in The Lightning Thief.

Personality
Nemesis is a cold hearted goddess, and isn't afraid to do radical exchanges with her own children, such as blinding Ethan in one eye so he will receive power instead. Percy comments that he wouldn't want a mother like her. However "Nemesis always pays her Debts, good or evil" says Ethan.