Opais is the daughter of Karata who lived on Sel during the Late Era.
When Karata was taken by the Shaod, no one explained to Opais what had happened to her mother. She lived with her father in the guard's quarters in Iadon's palace in Kae.
While in Elantris, the children Karata saw reminded her of Opais, so she made taking care of them the priority of her gang.
Karata attempted to sneak out of Elantris three times to find her daughter, but she was always caught before she managed it. When Karata was about to destroy New Elantris, Raoden offered to get her into the the palace in exchange for her leaving his people alone.
Raoden got her into the palace and Karata left a letter explaining what had happened to her, as well as a necklace that she had sneaked past the guards, next to a sleeping Opais.