Tasha Lem

 was the final Mother Superious of the Papal Mainframe, having led the organisation at the time of the Siege of Trenzalore. She was an old friend of the Doctor's and was "against" aging.

Biography
Tasha and the Doctor had a close relationship in their past, Tasha becoming comfortable enough with acknowledging his different bodies. She learnt to fly his TARDIS but would later note to Clara Oswald that she had "never quite mastered" flying the Doctor.

When a mysterious transmission began emanating from the planet Trenzalore, the Church responded to it, erecting a force field around the planet to protect the Level 2 human colony from the various species who had followed.

When the Eleventh Doctor arrived, Tasha summoned him and Clara aboard the Papal Mainframe. After a discussion with the Doctor about the transmission, she teleported him and Clara down to the planet to discover the source of the message. The two soon discovered that the message was from the Time Lords, lost in a pocket universe since the end of the Last Great Time War, seeking to return to N-Space via a time field. Contacting the Doctor, Tasha refused to allow him to help his people, noting that the marauding fleet would destroy Trenzalore to stop the threat of the Time Lords. When the Doctor announced his intent to defend both the planet and his people, Tasha declared that the Church would now dedicate itself to ensuring that the Doctor would never speak his true name, decreeing "silence will fall."

Over the next three centuries, the Kovarian Chapter broke away and travelled back along the Doctor's timeline to try and prevent him from ever reaching Trenzalore. Though Tasha did not take part in these schemes, she knew that they had ultimately caused the crack that the Time Lords were calling through and ensured that the Doctor would live long enough to reach Trenzalore.

Eventually the New Dalek Paradigm grew tired of the holding action and assaulted the Papal Mainframe, killing everyone and turning them into Dalek puppets. From Tasha's memories, the Dalek pathweb reconstructed its knowledge of the Doctor. The Daleks revived and tortured Tasha "several times" in an attempt to learn how to break the force field but she refused to reveal the information.

Instead, the Daleks used Tasha as a lure in a trap, bringing the Doctor back to the Mainframe where Tasha revealed her converted nature and took Clara as a hostage. Before the Daleks could exterminate them, the Doctor praised Clara's fearlessness when faced with them, denouncing Tasha as spineless and unreliable. The anger allowed Tasha to overcome the Dalek programming, regaining her free will and exterminating the Dalek drones. Before the programming could take over once again, Tasha teleported the Doctor and Clara back to the TARDIS, encouraging the Doctor to fly away now that he had his ship back, though insisting that she done everything for galactic peace, not the Doctor.

After a further five centuries of warfare against the New Dalek Paradigm, the Doctor, having used all twelve of his regenerations, was dying of old age. Feeling that the Doctor should not die alone, Tasha used the TARDIS to collect Clara Oswald so his companion could be with him in his last moments. Directing Clara to the Doctor, Tasha went off to join her forces in the final push against the Daleks. (TV: The Time of the Doctor)

Tasha Lem herself died during the Siege. As a sign of respect, she was buried with a TARDIS key. A fleet of morimentum ships were sent to collect a thousand tonnes of ice each. Over the next thirty years, her tomb was carved out in the ice, and after her burial it was defended by a battalion of Sacristans, assassin androids. She was the last Mother Superious.

Sometime after her burial, Sibling Same and Sibling Different stole the TARDIS key from her grave, and took it to London in the year 1895, where they ensured it fell into the hands of the Paternoster Gang. The Siblings hoped they would contact the Doctor. However, Vastra instead handed the key back to the Sacristan android that had been sent back to retrieve the key. (PROSE: A Perfect Christmas)

Behind the scenes

 * In 2017, Steven Moffat said that he had been frustrated with Tasha's main scene because of how flirtatious her interactions with the Doctor seemed. "What I was looking for was, it was like the Doctor meeting his first girlfriend, you know, and she's now the pope and he's now Doctor Who, so all that's out the window. So they have a certain knowledge of each other, but it's from a long time ago, and they're at ease. I think it went a bit too River Song, because everyone loved River so they sort of made her into another River."
 * According to Moffat in DWM 473, Tasha may have at one time been married to Madame Kovarian.
 * According to Orla Brady, Tasha is mostly human but has some alien ancestry as well.
 * Orla Brady appeared in character as Tasha in a video wishing Doctor Who fans a "killer Christmas".
 * Tasha is both a playable character and an enemy in the Doctor Who: Legacy mobile game.