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, following his inability to save Cass because she refused his help, owing to her hatred of the Time Lords. The Sisterhood revived the Doctor 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's body from the wreckage, the Doctor gave up hope and decided to enter the Time War. He drank her potion, which she said contained the Elixir of Life, (TV: The Night of the Doctor) but really was just lemonade and dry ice, (PROSE: The Day of the Doctor) and regenerated into a warrior for battle. (TV: The Night of the Doctor) Ohila's "moment of theatre" allowed the War Doctor to embrace what he needed to do without falling into self-loathing, (PROSE: The Day of the Doctor) as he abandoned the name of "the Doctor". (TV: The Night of the Doctor)

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, disgusted. The Doctor ultimately acknowledged that she was right. (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 Doctor Who head writer, executive producer and writer of The Night of the Doctor Steven Moffat, Ohila's name is a reference to Ohica.