Ashildr

Ashildr, portrayed by Maisie Williams, debuts in "The Girl Who Died" as a Viking puppet maker whose village is attacked by the Mire. The Doctor aids her defeat but she dies, prompting him to use a Mire repair kit to revive her, rendering her immortal.

In "The Woman Who Lived," 800 years later, Ashildr adopts the name "Me" and becomes a highwayman. Her immortality causes memory loss due to human brain limitations. Reuniting with the Doctor, she plans to leave Earth but betrays someone, ultimately helping close a portal. She vows to protect the Doctor's companions.

In "Face the Raven," Ashildr operates an alien refugee camp in London. Clara dies there as part of a plot involving Rigsy, leading to Time Lords' involvement. She is revealed as the hybrid destined to destroy Gallifrey.

In "Hell Bent," at time's end, Ashildr is humanity's last remnant. She aids the Doctor and Clara in resolving their memories. They part ways with Clara leaving in her TARDIS, continuing adventures while Ashildr extends her lifespan.

Casting details highlight Williams' enthusiasm for joining the series and praise from Moffat on her challenging the Doctor. Reception was largely positive, with acclaim for Williams' performance, though some remained unconvinced by Ashildr initially.