The High Priestess was a female Fabrini religious leader who led the Fabrini population living on Yonada and later on Lorina (Daran IV). She would serve as the representative of the Oracle of the People's will to Fabrini, and would consult with the Oracle for guidance.
In 2268, the High Priestess was a woman named Natira. She guided the people of Yonada during the final stages of the ship's voyage to Daran IV. After her instrument of obedience was removed, and learning that the Oracle was a nothing more than a sophisticated computer Natira gave up the office of High Priestess. (TOS episode: "For the World is Hollow and I Have Touched the Sky")