Thirteenth Doctor

The Thirteenth Doctor was the first female incarnation of the Time Lord known as the Doctor and the second incarnation to come from the Doctor's second regeneration cycle.

A day to come
When encountering the "Vortex Butterfly", the Tenth Doctor was cryptically told that he would not be "limited" to "thirteen lives". (COMIC: Vortex Butterflies)

When the Twelfth Doctor broke his toe, Clara Oswald suggested regenerating to heal the injury, but he berated the idea as a waste. (PROSE: The Blood Cell)

When threatened by Captain Lundvik, the Twelfth Doctor told her she would have to shoot him, Clara Oswald and Courtney Woods, but warned that she would "have to spend a lot of time shooting [him] because [he] [would] keep on regenerating." (TV: Kill the Moon)

While suffering from the common cold, the Twelfth Doctor, overreacting to the illness, considered the possibility of needing to regenerate. (COMIC: The Day at the Doctors)

When the Twelfth Doctor confronted Rassilon in Gallifrey's Drylands after escaping from his confession dial, Rassilon contemplated using his gauntlet to force the Doctor to regenerate as a method of torture, rhetorically wondering how many regenerations the Doctor had been granted, but was interrupted before he could attack the Doctor with the gauntlet. (TV: Hell Bent)

After the Monk invasion, the Twelfth Doctor needed to know if his companion, Bill Potts, was under the control of the Monks, and deceived her into shooting him in a rage to see if she had succumbed to the mind control, secretly putting blanks in all the guns, and faking his regeneration to complete the illusion. He made it look like the process had started, but emerged as himself to show her that he had deceived her. (TV: The Lie of the Land)

Post-regeneration
After the Twelfth Doctor was gravely wounded by the Cybermen on the Mondasian colony ship, the regenerative process began. However, tired of "being someone else", the Doctor delayed the change for several weeks, (TV: The Doctor Falls) until an encounter with his first incarnation, Archibald Hamish Lethbridge-Stewart, and the Testimony caused the Doctor to concede that another regeneration wouldn't "kill anyone". After taking a final look at the universe and providing advice to his next incarnation, the Doctor regenerated inside his TARDIS in an explosive fashion. (TV: Twice Upon a Time)

Noticing that her clothes no longer fitted, and feeling "there was something different about this body", (PROSE: Twice Upon a Time) the Doctor staggered to the console in a daze. Examining her face in a reflection, the Doctor saw that she had regenerated into a woman, and felt that the change was "brilliant". After she pressed a button on the console, the TARDIS suddenly spiralled into chaos, caused in part by the explosive regeneration. Subsequently, she was thrown out through the TARDIS doors in the confusion after the time rotor exploded, with the TARDIS itself vanishing without the Doctor as she fell towards Earth. (TV: Twice Upon a Time)

Companionless travels
The Doctor once posed as a museum curator in Venice. visited her to ask for the location of items that had been stolen from her in the 14th century. Missy took an old map from her and left. Later, the Doctor saved Antonia from being left behind in 14th century Venice, returning her to the present. She left Antonia with a note chastising Missy for her actions and that she would have to try harder next time. She then cleared out her office and told her assistant to tell anyone who asked that "the Doctor doesn't work here anymore." (PROSE: The Liar, the Glitch and the War Zone)

Sat on a bench in Henry VIII's third-favourite garden, the Doctor thought about the lesson the Moment had wanted to impart. The Moment joined the Doctor on the bench, and when the Doctor asked her why she helped him the day he saved Gallifrey, she said it was because she did not want to be used. Some time later, when the Doctor stood in a fountain at the heart of the Villengard banana groves, the Moment appeared again, continuing their conversation. The Moment told her that she helped because the universe needs the Doctor, and at that point he was in danger of stopping. Pleased that she finally had an answer, the Doctor went back to the TARDIS with a renewed vigor. (PROSE: The Day of the Doctor)

Undated events

 * The Doctor had an encounter with Alistair Gordon Lethbridge-Stewart that was recorded on video. (PROSE: Avatars of the Intelligence)
 * At some point before 5 March 2005, the Doctor was photographed "running away from a giant frog in front of Buckingham Palace". (PROSE: Rose)
 * At some point, she met Cass and gave her a bandolier. (PROSE: The Day of the Doctor)

Personality
The Thirteenth Doctor was not above being secretive and cryptic, handing an annotated map of 14th century Venice, without explaining it and leaving Missy to discover for herself what the annotations meant, while also not divulging her true identity to Missy. (PROSE: The Liar, the Glitch and the War Zone) She also demonstrated a playful sense of humour, labelling the Moment's interface as "a Christmas Cracker", and responding to Cass Fermazzi's mention of having childhood therapy with a dry "[don't] we all". (PROSE: The Day of the Doctor)

The Doctor claimed that she enjoyed making "trip[s] into the past" for research purposes, but knew to be cautious when making such trips. She retained the good nature of her predecessors, making a trip to 14th century Venice to rescue Antonia, and writing a note to chastise Missy for her lack of caution. (PROSE: The Liar, the Glitch and the War Zone)

She retained the guilt demonstrated in her prior incarnations over the events of the Last Great Time War. Visiting Cass prior to her death, the Doctor expressed her regret over Cass' fate, and noted that saving Cass was impossible as "[Cass] was too wrapped up in [her] timeline", demonstrating her continued respect for the Laws of Time via being unwilling to alter her own personal timeline. (PROSE: The Day of the Doctor)

Appearance
This incarnation of the Doctor resembled a woman in her mid-thirties, possessing jaw length blonde hair with dark roots and hazel-coloured eyes. (TV: Twice Upon a Time) Lucy Wilson described her as a "beautiful woman". (PROSE: Avatars of the Intelligence)

Clothing
Immediately after her regeneration, the Thirteenth Doctor wore her predecessor's battle-worn coat, (TV: Twice Upon a Time) but eventually wore braces, something that Rose Tyler commented on when shown a photograph of her by Clive Finch. (PROSE: Rose)

First female Doctor
The Thirteenth Doctor is the first and,, only incarnation of the Doctor in the show's history to be played by a woman. Before Jodie Whittaker, though, the idea of a woman Doctor had been explored.

The idea that a female actor could take the role of the Doctor was first publicly introduced by John Nathan-Turner and Tom Baker in 1980. By Baker's suggestion, he told the press, "I certainly wish my successor luck, whoever he—or she—might be." Peter Davison was cast as the Fifth Doctor, but the idea remained alive.

By the time Whittaker was announced as the actor to follow Peter Capaldi, it was well-established, to viewers and within the Doctor Who universe, that the Doctor could be played by a woman.

Though a parody, The Curse of Fatal Death (1999) by Steven Moffat introduced another Thirteenth Doctor, played by Joanna Lumley. This Doctor, upon regenerating, immediately noted that she had "etheric beam locators". When Emma tells her that those are actual breasts, the Doctor says that she "always wanted to get [her] hands on one of these". At the end, she suddenly finds the Master attractive and walks off with him, arms around each other's waists.

The Big Finish Doctor Who Unbound story Exile (2003), though not set in the prime Doctor Who universe, starred a female Third Doctor, played by Arabella Weir. The story tried to establish that, in this universe, suicide was necessary for a "sex-change regeneration", which was also considered a crime by the Time Lords. Later stories, particularly in the Steven Moffat era, would contradict the idea that changing gender during regeneration was anything out of the ordinary.

In 2010, The End of Time: Part Two had the Eleventh Doctor briefly think he had regenerated into a female, immediately post-regeneration. He quickly realised that he was mistaken, on finding his Adam's apple.

In 2011, Neil Gaiman's The Doctor's Wife was the first to make direct reference to a Time Lord changing gender through regeneration. The Doctor talks of the Corsair, a "fantastic bloke", who was also a "bad girl" in a couple of their incarnations.

In 2013, The Night of the Doctor had the Sisterhood of Karn offer the Eighth Doctor the choice of "man or woman", for his approaching regeneration.

In 2014, Steven Moffat introduced audiences to, revealed in Dark Water to be the first female incarnation of the Master. Missy was a recurring character in Series 8, 9, and 10.

In 2015, Hell Bent showed audiences the regeneration of the General from a male body to a woman, the first time such an instance occurred on-screen, though the Big Finish audio story, The Black Hole, had a male to female regeneration eighteen days before the episode aired. A year later, Enemy Lines showed the first female to male regeneration.

In 2017, World Enough and Time included a rooftop conversation, between the Twelfth Doctor and Bill, in which he's only "fairly sure" that his first incarnation was a man, as it was a long time ago. The Twelfth Doctor here claims that Time Lords are "beyond [the] petty human obsession with gender and its associated stereotypes".

The same year, the audio story The Conscript included a conversation between the Eighth Doctor and a Time Lord soldier, where the Doctor stated that he was a "he, for now at least."