Maren

 Maren of the Sisterhood of Karn was the high priestess of the Temple of the Flame on the planet Karn and, with the other priestesses of the Sisterhood, guarded the Sacred Flame and the Elixir of Fire the flame produced, which could grant immortality. Maren generally distrusted the Time Lords.

During the war against the renegade Time Lord Morbius, as Reverend Mother, Maren advised the Hospice of Karn and had authority over who might receive a sip from the Elixir to heal them. Later she would gather the Sisterhood to assist the Doctor's vast multi-species army the Alliance to pool together the priestess's psychic powers to defeat Morbius. (PDA: Warmonger) After Morbius' trial, Maren watched his execution, little guessing at the time that his brain survived the destruction of his body. (DW: The Brain of Morbius)

Decades later, the Doctor encountered Maren for the first time, though, in the Maren's personal timeline the two had already met. (The Doctor had used an alias at that time.) (PDA: Warmonger) The Sacred Flame appeared to be dying, she suspected the Doctor of attempting to steal the remaining Elixir. Later, she gave up her ration of Elixir to save the Doctor's life, and died in the Sacred Flame. Maren's most trusted priestess, Ohica succeeded her. (DW: The Brain of Morbius)