Fourteenth Doctor

The Fourteenth Doctor was the result of his predecessor's solitary regeneration, who was fatally wounded during her final fight with to save the Earth from destruction. He was mystified by his 'new' body and appearance, given his close resemblance 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. (TV: The Day of the Doctor)

The Twelfth Doctor guessed that he may "keep on regenerating forever" if he was shot by Captain Lundvik threatened to shoot him. (TV: Kill the Moon) Indeed, while threatening him on Gallifrey, Rassilon was unsure as to how many regenerations the Time Lords granted the Doctor. (TV: Hell Bent) and  were also unsure, pondering if they "could [be] up and down the stairs all night" if they were to throw the Doctor off the roof of a building on a Mondasian colony ship. (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)

A new body
After the Thirteenth Doctor and her allies foiled ’s plot to possess the Doctor's body, 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 Thirteenth Doctor, fatally wounding her. Staving off regeneration long enough to have a final conversation with Yasmin Khan, she then travelled to a cliff alone overlooking the sea, where she could watch the sunrise one last time.

After saying her farewells to her current incarnation, she 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 Doctor found himself taken aback by the fact that he was in a body all-too-familiar, noting his teeth and repeatedly exclaiming "what?" (TV: The Power of the Doctor)

Psychological profile
to be added

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 paisley tie, a loose fitting tartan waistcoat and trousers. He also wore a navy blue trenchcoat and Converse shoes. He greatly resembled (TV: The Power of the Doctor) his tenth incarnation, with the Doctor recognising this form by feeling 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 a news article posted to the official Doctor Who website following the episode'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 and The Curator (TV: The Day of the Doctor), Colin Baker who played the Sixth Doctor, another form of The Curator (AUDIO: Stranded 4), and an alternate Fifth Doctor (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.