A day to come
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 the Saxon Master and Missy 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)
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)
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.