Thirteenth Doctor (The Curse of Fatal Death)

The Thirteenth Doctor was the result of the regeneration of the Twelfth Doctor.

Biography
The Thirteenth Doctor came about after 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 their previous plans to marry her were cancelled, as the Doctor was no longer "the man [Emma] fell in love with". However, the Doctor and found each other much more attractive than before and walked off together. (TV: The Curse of Fatal Death)

Behind the scenes

 * In an early draft of the script, written prior to casting, the thirteenth incarnation was referred to as "The Lady Doctor", and described only as "a beautiful woman". Her final name, used in the behind-the-scenes documentary and in Doctor Who Magazine credits, was "The Female Doctor".
 * 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 with the release of Twice Upon a Time, eighteen years after Lumley's Doctor had her appearance in The Curse of Fatal Death.
 * 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.
 * Lawrence Miles's novel pitch The War, as well as at least one other pitch by David A. McIntee, would have featured this incarnation of the Thirteenth Doctor.