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)
The inclusion of a woman as the Thirteenth Doctor addressed long-standing fan speculation that the Doctor could at some point change sexes while regenerating. The theory has been supported by Tom Baker. The possibility of such a sex change had been suggested by the newly regenerated Eleventh Doctor at the conclusion of The End of Time when he found he had long hair and momentarily thought he was now "a girl". The first direct confirmation that a Time Lord can change sex later came in TV: The Doctor's Wife, where the Doctor mentioned that he had known the Corsair in various male and female incarnations.
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".