Fourteenth Doctor

The Fourteenth Doctor was the result of his predecessor's solitary regeneration, having been fatally wounded during her final fight with to save the Earth from destruction. He was mystified by his 'new' body and appearance, given he looked virtually identical to the Tenth Doctor.

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)

The Curator had told the Eleventh Doctor that he might find himself revisiting old faces, “but just the old favourites.” (TV: The Day of the Doctor)

Whilst facing the Daleks on Trenzalore, the Eleventh Doctor, who had run out of regenerations, was granted a new regeneration cycle by the Time Lords following a plea by Clara Oswald. (TV: The Time of the Doctor)

The Twelfth Doctor guessed that he may "keep on regenerating forever" in response to Captain Lundvik threatening to shoot him. (TV: Kill the Moon) Indeed, during his scuffle with the Doctor on Gallifrey, Rassilon was unsure how many regenerations the Time Lords granted the Doctor. (TV: Hell Bent) and  were similarly unsure, darkly joking they’d be "up and down the stairs all night" trying to kill the Doctor by repeatedly throwing them off a rooftop. (TV: The Doctor Falls)

The Thirteenth Doctor remarked to Cleo Proctor that she would stand up again if she "clobber[ed]" her, remarking that she would regenerate "Possibly several times" and then "who can say?" as to how many times. (AUDIO: Salvation)

Trying to generate enough energy, the Thirteenth Doctor once considered regenerating and using residual artron energy. (PROSE: The Maze of Doom)

The Thirteenth Doctor wrote a book about all of her lives, also using material written by other incarnations from the past and her fam, to aid the new Doctor if they "[felt] weird" after regeneration. (PROSE: A Short History of Everyone) However, the Doctor found herself in a regeneration-based crisis when, during his plot with the Cybermen and Daleks to defeat her once and for all, made her undergo a forced regeneration into himself to tarnish the Doctor's name. Finding herself trapped in her mind, the Thirteenth Doctor encountered vestiges of her past selves known as the Guardians of the Edge, with the manifestation of the Sixth Doctor remarking that they needed to stop the Master because the Doctor's title and identity was "supposed to be handed over" to whoever "the next one" would be. (TV: The Power of the Doctor)

A new body
After the Thirteenth Doctor and her allies foiled the Spy Master's plot and reversed her forced regeneration ‘into him’, the Master decided that if he couldn't be the Doctor, then neither could she. He spitefully exacted his revenge in his dying moments; using his Tissue Compression Eliminator, the Master directed the Qurunx's destructive blast at the Doctor, fatally wounding her. Staving off regeneration long enough to have a final conversation with Yasmin Khan, she then travelled alone to a cliff overlooking the sea, where she could watch the sunrise one last time.

After saying her farewells to her current incarnation, the Doctor began to regenerate; however, the regeneration not only replaced every cell in her body, but, unusually, caused her outfit to morph into a completely different set of clothes. After the regeneration was complete, the new Doctor was taken aback when he realised he possessed a body all-too-familiar; he noted his teeth and new outfit, prompting him to repeatedly exclaim "what?!". (TV: The Power of the Doctor)

First adventures
After settling back into his old body surprisingly fast, the Doctor returned to the TARDIS and allowed it to take him to Wembley Stadium during the 1966 World Cup Final, in pursuit of a distress signal. While looking to see if anyone was in distress, and due to some faulty disguises, he found a group of purple aliens in the stadium as time tourists, who were present to watch a historical football tournament when a Dalek flying saucer arrived in the stadium to deploy a force of Bronze Daleks and begin another invasion of Earth. (COMIC: Liberation of the Daleks)

Psychological profile
Upon gaining a body almost completely similar to his tenth incarnation, the Fourteenth Doctor also regained a number of that incarnations psychological traits. Indeed, his immediate response to discovering that he had returned to his former body, coupled with the unexplained appearance of a new outfit, led to him repeatedly uttering "what?" in his bewilderment. (TV: The Power of the Doctor)

Nonetheless, the Doctor appeared to settle back into his old body very quickly, with no signs of the post-regenerative trauma that his predecessors suffered. Indeed, his first act post-regeneration was to re-enter his TARDIS, bursting with enthusiasm and curiosity at the distress signal that it had just received. (COMIC: Liberation of the Daleks)

Appearance
The Fourteenth Doctor, upon regeneration, wore a costume distinct from his predecessor's ensemble. His attire included a white button-up shirt, with a tie, a loose fitting tartan waistcoat and trousers. He also wore a navy blue trenchcoat and Converse shoes. He greatly resembled a slightly older version (TV: The Power of the Doctor) of his tenth incarnation, with the Doctor recognising this form by the feeling of his teeth and face. (TV: The Parting of the Ways, The Christmas Invasion, et al.)

Behind the scenes

 * This Doctor's numbered designation was officially confirmed by Russell T Davies in an article posted to the official Doctor Who website following the Centenary Special's broadcast.
 * This in turn makes David Tennant the fourth actor to play multiple separate incarnations of the Doctor, along with Tom Baker who portrayed the Fourth Doctor from TV: Robot to TV: Logopolis and the Curator in TV: The Day of the Doctor, Colin Baker who played the Sixth Doctor from TV: The Caves of Androzani to The Ultimate Foe, another form of the Curator in AUDIO: Stranded 4, and an alternate Fifth Doctor in AUDIO: Doctor of War, and Richard E Grant, who played versions of the Tenth and Ninth Doctors in The Curse of Fatal Death and Scream of the Shalka respectively.
 * Though Sylvester McCoy briefly played the Sixth Doctor at the beginning of TV: Time and the Rani and Paul McGann briefly played the War Doctor at the end of TV: The Night of the Doctor, they were only standing in for these incarnations' primary actors.
 * Beyond the post-regeneration scene in The Power of the Doctor, this incarnation's first appearance was as a character-skin in the free multiplayer battle royale video game Fall Guys, released on 1 November 2022.
 * The Fourteenth Doctor is the first and only incarnation to have their initial post-regeneration story depicted via a comic: Liberation of the Daleks, published in Issue 584 of Doctor Who Magazine.
 * The Fourteenth Doctor is the first incarnation since the Second Doctor to emerge from their regeneration with a new outfit. This deviation from the usual norm of the newly-regenerated Doctor still donning the clothes worn by their predecessor led to some confusion among viewers.
 * Russell T Davies clarified that "[he] was very certain that [he] didn’t want David to appear in Jodie’s costume." explaining that his reason for the Doctor's clothes changing during regeneration was to avoid stereotyping "the notion of men dressing in "women's clothes", the notion of drag", describing it as a "very delicate" matter, expressing that "it has to be done with immense thought and respect", and that the media would make it "look like mockery" of that culture, as David Tennant is taller than Jodie Whittaker.