Ohila

Ohila was the High Priestess of the Sisterhood of Karn and was active during the Last Great Time War, as well as the aftermath of the war's end.

Biography
Ohila knew the Doctor, with an implication of her knowing him as far back as his first incarnation's childhood. (TV: Hell Bent) She believed that he had been a warrior for his entire life. (PROSE: The Day of the Doctor)

The Last Great Time War
Ohila was pleased to see the Eighth Doctor return to Karn, but pitied the fact he had been killed in a crash landing. The Sisterhood revived him for four minutes, and Ohila begged him 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 Fermazzi's body from the wreckage, the Doctor gave up hope and decided to enter the Time War. He drank the elixir which Ohila had taken the liberty of preparing, and he regenerated into a warrior for battle. (TV: The Night of the Doctor) In actuality, the elixir was really just lemonade and dry ice, with Ohila putting on a "moment of theatre" to help the Doctor embrace what he needed to do without falling into self-loathing before he regenerated from injuries he had sustained in the crash. (PROSE: The Day of the Doctor)

Ohila led the Sisterhood when they joined the Time Lords in fighting the Daleks. Alongside the War Doctor and Fey Truscott-Sade, they fought the Morlontoa of the Seventh Sky. The Sisterhood uttered many incantations that held back the creature's spores. During the battle, the Sisterhood were struck by a stream of pure chaos, leading the natives of the planet, the Loshann, to be hit by the spores. The Loshann were quickly inverted, turned into monsters and attacked. To save themselves and other Loshann on the planet, the Doctor ordered Fey to shoot each of the Loshann. After she did so, the Doctor's machine to save the planet failed, meaning the planet would fall anyway and Fey had killed them for nothing.

While the planet was saturated by the spores, the Doctor ordered Ohila and Fey back to the TARDIS. Fey saw a child in the wreckage and saved him, but wasted time she didn't have. Fey and the child were touched by the spores and began to die. The Doctor and Ohila, unable to save them, watched from the TARDIS as Fey melted away. Ohila told the Doctor that he did all that he could. He snapped at her, telling her not to call him by his name again. (COMIC: The Clockwise War)

After the Time War
Ohila met the Doctor again in his twelfth incarnation. He was present on Karn when Colony Sarff arrived and spoke to Ohila about searching for him on behalf of Davros. Sarff gave a message to Ohila to pass on to the Doctor (unaware he was listening), stating that "Davros remembers", and an invitation to meet him one last time. (TV: The Magician's Apprentice) She didn't believe his answers when discussing the invitation, 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. Before leaving Karn, the Doctor gave her his confession dial, saying that she'd know who to give it to. (WC: Prologue) She later gave it to. (TV: The Magician's Apprentice)

When Ohila heard the Twelfth Doctor had returned to Gallifrey, she went there and met with the General and the High Council, much to the Lord President's irritation. Ohila watched Rassilon's efforts to reach out to the Doctor with amusement and told Rassilon that the Doctor didn't blame Gallifrey for the horrors of the Time War, but rather Rassilon himself. After Rassilon was overthrown, Ohila spoke with the Doctor about his actions and about the Hybrid. She later consulted with the General about the Doctor's actions and, deep in the Cloisters, told Clara what happened in the Doctor's confession dial and why. As the Doctor and Clara escaped in a stolen TARDIS, Ohila yelled that he was going against everything he ever believed in before leaving. The Doctor ultimately acknowledged that she was right. Ohila watched his departure with disgust, commenting that the Doctor was going to do what he does best: run. (TV: Hell Bent)

Alternate timeline
Ohila was present when the Twelfth Doctor crash landed on Karn. He asked Ohila to take him to the Sisterhood's back door to Gallifrey. She was reluctant at first, but after the Cybermen began to attack the Sisterhood, she showed him to the door and sealed it behind him when he passed through. (COMIC: Supremacy of the Cybermen)

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