Ashildr

Ashildr, also known as Lady 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, encountering the Doctor again hundreds of years later.

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, ruining 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
After eight hundred years of being immortal, 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, 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, who was also 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)

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