Thirteenth Doctor (The Curse of Fatal Death)

The Thirteenth Doctor was an incarnation of the Doctor, the result of the regeneration of the Twelfth Doctor.

Biography
Her previous incarnation regenerated after being hit by zectronic energy. This was unexpected as those present at the death of the Twelfth Doctor assumed that the zectronic energy cancelled the regeneration process.

With the Doctor now regenerated into a woman, she was abandoned by her companion and lover Emma and "his" previous plans to marry her were cancelled. However, the Doctor and the Master found each other much more attractive than before and walked off together as a couple. (NOTVALID: The Curse of Fatal Death)

Behind the scenes

 * The Thirteenth Doctor from the main universe, introduced in Twice Upon a Time, was, in fact, a woman. She was played by Jodie Whittaker, 18 years after Lumley's Doctor had her appearance in The Curse of Fatal Death.
 * The inclusion of a woman as the Thirteenth Doctor addressed long-standing fan speculation, at the time, that the Doctor could at some point change sexes while regenerating. The theory had been supported by Tom Baker. During the Steven Moffat era, the Master and the General both changed genders during regeneration. Finally, the Thirteenth Doctor herself was a woman, ending the debate once and for all in 2017.
 * Fans had for years thought Joanna Lumley a candidate for the role of the Doctor, should they ever cast a woman in the role. She had earlier played Sapphire, a non-human time detective in the cult fantasy series Sapphire & Steel. The script gave her name as "The (Female) Thirteenth Doctor".
 * Another female incarnation of the Doctor would later appear in AUDIO: Exile, played by Arabella Weir.