The Doctor (Contents)

One incarnation of the Doctor was the product of a regeneration of an incarnation postdating the Twelfth and Thirteenth 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)

When Captain Lundvik threatened to shoot him, the Twelfth Doctor speculated that he would "keep on regenerating forever" if he was executed. (TV: Kill the Moon) Rassilon would later state his own uncertainty to the number of regenerations the Doctor had been granted, (TV: Hell Bent) with and  also considering their own uncertainty when debating whether to throw the Twelfth Doctor off a roof, believing they "could [be] up and down the stairs all night." (TV: The Doctor Falls)

The Doctor told Cleo Proctor that, had she "clobber[ed]" her, she would only get back up again. "Possibly several times, who can say?" (AUDIO: Salvation)

Post-regeneration
Immediately after the regeneration, the Doctor decided that the attire of their previous incarnation "wo[uld]n't do". The Doctor then chose their new clothing in the TARDIS, sturdy boots and a new hat being amongst the items chosen. After this, the Doctor exclaimed "And that is that". (PROSE: Contents)

Salvation of Gallifrey
On the final day of the Last Great Time War, the Twelfth Doctor claimed that the entirety of the Doctor's original life-cycle plus himself had united above Gallifrey to save it from the Dalek Empire and end the War, (TV: The Day of the Doctor) but he also told the Eleventh General that, in truth, he was not sure how many incarnations were flying their TARDISes in the war-torn skies, only being certain it was enough to create "a blizzard" of blue-boxes; (PROSE: The Day of the Doctor) by one account, every incarnation of the Doctor had gathered to save their homeworld, which would mean (PROSE: Dalek Combat Training Manual) the Doctor who postdated the Thirteenth Doctor (PROSE: Contents) had joined the effort. (PROSE: Dalek Combat Training Manual)

Appearance
This incarnation had a "nice" new face which fitted well into place. Their hair was short and their nose was just right. Their legs were firm and their eyes were bright.

Wearing the clothes of their predecessor for a short time and believing them to be "off-the-peg", the Doctor first pondered on wearing a frilly shirt and a scarf before deciding "Think I'm over them" and "A touch too soon" respectively. The Doctor finally chose some sturdy boots, believing them to be handy for the Moon and a "nice" new hat. (PROSE: Contents)

Behind the scenes

 * The gender of this incarnation is not made clear.
 * Although it is not explicitly stated exactly where Contents falls within the Doctor's life it depicts a chest with an item owned by each known incarnation by that point --excluding the War Doctor-- but including a handbag presumably owned by the female incarnation from The Death List, thus implicating this incarnation to be the one that follows her. Russell T Davies also mentioned the handbag in an interview to Radio Times regarding references in his illustrations to Jodie Whittaker's then yet-to-debut Thirteenth Doctor.
 * In TV: The Power of the Doctor the thirteenth incarnation was forcibly regenerated into a version of . Like the incarnation from Contents, the Master's incarnation chose their new costume in the TARDIS by searching through the Doctor's back catalogue of clothing items. Their new outfit consisted of - from Contents - a recorder, a scarf and a celery lapel.
 * The Fourteenth Doctor was revealed in The Power of the Doctor to be portrayed by David Tennant and therefore identical to his tenth incarnation. This Doctor's clothes regenerated along with him and as such he did not choose his new outfit, ruling him out as the incarnation from this story.