Ashildr

Ashildr, also known as Lady Sherade, Lady Me and later Mayor Me and by the alias the Knightmare, was a Viking girl encountered by the Twelfth Doctor and Clara Oswald. After a battle with the Mire, Ashildr died, but she was resurrected by the Doctor via a Mire medical kit. As a result of this, she became immortal, surviving to the end of everything, witnessing the remainder of human history and remaining immortal even at the very end of time itself.

After an ordeal with the Time Lords at the end of the universe and returning the Doctor to his own TARDIS, she subsequently began travelling around in another stolen TARDIS with Clara.

As Ashildr
Ashildr was the daughter of Einarr, a farmer in a 9th century Viking village.

Ashildr greeted a group of Viking warriors when they returned from their journey with the Doctor and Clara in chains. Soon after their arrival, a figure claiming to be Odin appeared in the sky and invited the warriors to join him in "Valhalla".

Having been given half of the Doctor's sonic sunglasses by one of the warriors, and encouraged by Clara to use them, she was also taken to the Mire spaceship when undoing Clara's chains.

Unable to save the warriors, who were killed for their testosterone and adrenalin, Ashildr confronted "Odin" to the point of declaring war on the Mire. She and Clara were returned to the village, where they were given one day to prepare for the Mire's return.

When the Mire attacked, Ashildr was an integral part of the Doctor's plan to save the village, donning a Mire helmet and using her imagination to send false signals to the rest of the Mire, who viewed their surroundings via holograms. When "attacked" by a wooden dragon, the Mire believed the dragon to be real, and retreated when the Doctor threatened to share the footage which would ruin their reputation.

Ashildr was killed in the process, but was resurrected by the Doctor altering a Mire medical kit to fix human physiology. He left another for her to give to whomever she chose, knowing that she would continually be fixed, thus losing the "ability" to die. The Doctor later realised that she was a human-Mire hybrid. (TV: The Girl Who Died)

Immortality's burdens
During eight hundred years of immortality, Ashildr soon forgot her own name and even her life as a Viking. She began to pick up new names as her life went on, but each name died along those who knew her by it. She at some point decided to go by the name Me. As "Me", she had no attachment to anything or anyone that she would eventually outlast.

She took part in the Battle of Agincourt, declaring to be able to "fire six arrows in under a minute". She began to write a journal of her life, though she ripped out pages when reading them as they were too hurtful to force herself to remember in the future. On one occasion, she fell in love with a man before leaving him. She returned long after to find him aged to senility, and was horrified as he considered her a ghost of a time long gone. After losing her children to the Plague in the 14th century (TV: The Woman Who Lived, PROSE: The Triple Knife), she kept the pages in to remind her to never have anymore offspring as they would all die eventually. All this time, she kept the spare medical kit the Doctor gave her on a chain around her neck, unable to find anyone worth saving.

Soon becoming bored of life, she turned to living a double life as a criminal, stealing the riches from wealthy people while also living as "Lady Me" with a servant, Clayton, whom she gradually became bored of.

One day, Me found a Leonian named Leandro resting in her courtyard. He claimed to have crashed on Earth and they formed a partnership in order to retrieve the Eyes of Hades. The amulet would allow Leandro to leave Earth and he agreed to take Me with him.

While committing a robbery on a rich duo (during which time she affected a convincing male voice as part of her disguise), Me encountered the Doctor again while he was searching for an ancient artifact. The Doctor initially didn't recognise her, but did so after she removed her mask and reverted to her original voice. (TV: The Woman Who Lived)

Life in the 21st century
"Me" lived into the 21st century. She appeared in the background of a selfie taken by Clara Oswald and Evie Hubbard, which Clara planned to show the Doctor. When she did, he noticed Ashildr in the background but did not say anything to Clara. (TV: The Woman Who Lived)

Me became the mayor of an alien refugee camp hidden in the heart of London, for those who were desperate and destitute, without hope or a home. This camp was called a trap street. Later, to ensure the safety of the trap street, (TV: Face the Raven) she was convinced by the Time Lords to trick the Doctor into being transported inside his confession dial. (TV: Hell Bent) She set up a plot where Rigsy "murdered" Anah, a Janus resident of the trap street, when actually she was placed inside a stasis chamber by Ashildr. She placed a chronolock on Rigsy, and was successful in luring the Doctor to the street, where Clara "stupidly" took the chronolock from Rigsy. Horrified by this turn of events, Me revealed she couldn't remove the chronolock – she had made a specific agreement with the Quantum Shade for Rigsy, and Clara had destroyed the deal, taking it out of Me's hands. Clara was killed by the Quantum Shade, and the Doctor was tricked into having a teleport bracelet attached to his wrist to teleport him away. As he left, the Doctor warned Me that Clara's pleas were meant to protect her and her people from the Doctor, and it would be a good idea for Me to stay out of his way. Me acknowledged this numbly. (TV: Face the Raven)

At the end of the universe
Me was present at the very end of time, the last living being in the universe. To survive she created a bubble of reality among the ruins of the Cloisters on Gallifrey, passing the time watching the stars die while awaiting the Doctor and Clara's arrival. Five minutes before the final end of time, their stolen TARDIS appeared and Me knocked on the door.

Me had set up a chess game, though she and the Doctor didn't play. She remarked that the dying stars were a beautiful sight; when the Doctor said that it was sad, Me retorted that it was both, and the same was true of Clara's death, now a fixed point in time. She inquired about the Hybrid, which the Doctor accused her of being. Me countered that she was a human with a little bit of Mire, not a true hybrid. She then suggested that the Doctor might be half-human and thus he was the hybrid. As the Doctor refuted that suggestion, Me offered her true theory: the "hybrid" wasn't a single individual, but a Time Lord and his human companion, so alike that they constantly push each other into more and more catastrophic actions.

The Doctor revealed he had come to the same conclusion and was thus planning to erase all Clara's memories of him and leave her someplace safe. Me then entered the Doctor's stolen TARDIS and witnessed his final confrontation with Clara in silence. (TV: Hell Bent)

Travelling with Clara
Me assisted Clara in her attempt to make sure the Doctor's memory of her was truly gone, but remained in the new TARDIS so as not to arouse suspicion. When Clara was satisfied that the Doctor's memories were gone, she and Me left aboard the new TARDIS. Me reported that she was unable to get the chameleon circuit to work, so the TARDIS was stuck looking like a 20th century American diner. Clara approved, stating the situation was awesome. Clara then declared her intention to return to Gallifrey to face her death, but noted that since time didn't appear to be in any immediate danger and she was now ageless, there wouldn't be any harm in taking the long way back. Clara then activated the TARDIS, taking Me with her to explore time and space. (TV: Hell Bent)