Who is Ashildr, and how does she become a recurring character on the show?

Ashildr is a fictional character in the British science fiction television series Doctor Who. She is a Viking girl who is brought back to life by the Doctor using alien technology, making her immortal. She first appears in the episode 'The Girl Who Died' in series 9 of the show, played by Maisie Williams.

Later in the series, she becomes a recurring character when she reappears in 'The Woman Who Lived', and then again in 'Hell Bent'. In 'The Woman Who Lived', it is revealed that Ashildr has been living for over 800 years and has become a highwayman, stealing from the rich to give to the poor. In 'Hell Bent', she helps the Doctor return to his own timeline by giving him a new regeneration cycle.