Ohila

Ohila was the High Priestess of the Sisterhood of Karn, and was active during the Last Great Time War.

She was pleased to see the Eighth Doctor return to Karn, but pitied the fact he had been killed in a crash landing, following his failure to save Cass because she hated Time Lords. The Sisterhood revived him for four minutes, and Ohila begged the Doctor to help them by taking the Elixir of Life. After arguing with him, she convinced the Doctor that he was the only one capable of stopping the war, and he could not help anyone by running away from it just to uphold his principles as the Doctor.

Once the Sisterhood retrieved Cass's body from the wreckage, the Doctor gave up hope and decided to enter the Time War. He used her potion containing the Elixir of Life to regenerate into a warrior for battle, no longer going by the name of "Doctor". (TV: The Night of the Doctor)

Ohila met the Doctor again, this time in his twelfth incarnation, when he returned to Karn, offering him an invitation on behalf of someone else. She also expressed her great distrust of him, stating that it was easier to assume he was lying to her rather than trust his word. She also indicated that she considered herself and the Doctor to be friends, an assertion he didn't refute. The Doctor gave her a curved disk of unknown purpose as he left, saying that she'd know who to give it to. (WC: Prologue)

Behind the scenes
According to an interview with Steven Moffat, Ohila's name is a reference to Ohica.