Ashildr

Ashildr, later 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 of heart failure, but she was resurrected by the Doctor via a Mire medical kit.

Early life
Ashildr was the daughter of Einarr, a farmer in a Viking village. (TV: The Girl Who Died)

Declaring war
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, 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. (TV: The Girl Who Died)

Becoming immortal
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)

Life after the Doctor
After 800 years of being immortal, Ashildr soon forgot her own name and even her life as a Viking. She began to refer to herself as "Me". Even when reminded of her original name, she did not want to go by it anymore. As "Me", she had no attachment to anything or anyone that she would eventually outlast. She took part in the Hundred Years' War, declaring to be able to "fire six arrows in under a minute". She suffered great loss, losing children and lovers alike. After losing her children to the plague in the 15th century, she began to write a journal of her life, though she ripped out pages when reading them as they were too hurtful. She kept the memory of losing her babies to remind her to never have anymore. Unable to find anyone worth saving, she kept the spare medical kit the Doctor gave her on a chain around her neck.

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

One day she found a lion like alien resting in her courtyard who claimed to have crashed on Earth. She soon formed a partnership in order to retrieve an artifact to return the creature to space, with the promise he'd take her with him. (TV: The Woman Who Lived)

Reuniting with the Doctor
While committing a robbery on a rich duo, Ashildr 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)