The Curator

The Curator was an elderly incarnation of the Doctor who had retired from his adventurous ways to become "a humble curator". He curated the Under-Gallery, where he watched over the painting Gallifrey Falls No More after acquiring it under "remarkable circumstances."

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 Eleventh Doctor's timeline became distorted, the Curator was seen as one of his future incarnations, at the end of his lifespan. (COMIC: The Then and the Now)

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 have been up and down the stairs all night." (TV: The Doctor Falls)

Reunions
Many years after the Eighth Doctor had regenerated on Karn, the Curator asked Ohila what had been in his regeneration potion that supposedly contained the Elixir of Life. She told him it actually contained dry ice and lemonade and went on to explain that the dark nature of the Doctor had always been inside him and he just needed to pretend it had come from somewhere else. The Curator and Ohila were good friends by this point; he reminisced later about her "always [being] such a beautiful woman, and a thrilling and innovative darts player".

During one of his many visits to Alistair Gordon Lethbridge-Stewart, the Curator asked him what the most important quality in a UNIT commander was. He responded with the answer of good handwriting. The Curator and Alistair got up to a lot of "mischief" and laughed with each other until he died. One activity they did was play Risk together. According to the Curator, Alistair was always the Daleks, which the Curator found most unfair. (PROSE: The Day of the Doctor)

Semi-retirement
Under "remarkable circumstances," the Curator was able to acquire Gallifrey Falls No More and brought it to the Under-Gallery. (TV: The Day of the Doctor)

On another of his visits to see the Brigadier, he told him he was planning to write a book about the last day of the Last Great Time War. He told him that information within it was classified material to which he responded he had a plan to mark the book as fiction. They had a brief argument before Alistair started to ask questions about why the Doctor Papers existed. The Curator proceded to answer them before a toy Dalek pinged off the window next to his head. (PROSE: The Day of the Doctor)

The day of the Doctor
At around the time the time fissure appeared in the Under Gallery, the Curator was in the middle of moving some of the more dangerous artifacts in the Under Gallery to the Black Archive marking the empty display cases and stands with a "B".

For his book about the Last Great Time War, the Curator compiled the Doctor Papers together in order to make comments about them. The sections where the Curator talked to the reader, however, were written live and ten years in the future because he got sloppy about the deadline. Towards the end of the book, the Curator got his webcam working, meaning it would automatically translate what was going on into prose. As a treat, he set off to find his past incarnations and discover the identity of a mysterious stranger who met with him, possibly from the Doctor's past or future. (PROSE: The Day of the Doctor)

When his war, tenth and eleventh incarnations rendezvoused at the National Gallery after the final battle of the Last Great Time War, the Curator approached Clara Oswald, telling her he was looking for the Doctor. (TV: The Day of the Doctor) Another account suggested he accidentally alerted the Eleventh Doctor to his presence by speaking to himself around the corner at which point he decided to get involved in the narrative, even though it was strictly against rules. (PROSE: The Day of the Doctor) When the Eleventh Doctor was alone, the Curator made his presence known when the Doctor noted how he could retire and be a curator, with the Curator exclaiming that he "really [thought] [the Doctor] might." Stunned by how familiar the Curator was, the Doctor noted that he "never forget[s] a face", the Curator merely noted that the Doctor would revisit a few in years to come but just the "old favourites".

He told the Doctor that the name of the painting was neither No More nor Gallifrey Falls, but in fact one title Gallifrey Falls No More. Despite proclaiming to be a "humble curator [who] wouldn't really know", he told the Doctor that Gallifrey was lost, and pointed him in the direction to search for it, telling him he had "a lot to do" and congratulated the Doctor. When the Doctor tried to pry into his meaning, the Curator retaliated by pointing out it was the Doctor's choice. He was then about to tell the Doctor what he would do if he was him, but then teasingly said that "perhaps [he] was [him]," or that maybe the Doctor had been him, before deciding that it "[didn't] matter either way" and left. (TV: The Day of the Doctor)

Appearance
Having the appearance of an elderly Fourth Doctor, the Curator's face was wrinkled with baggy cheeks, a hooked nose and a pronounced throat. He had light blue eyes and big ears with dangling ear lobes.

Though he was balding, his hair was completely white, and he had no eyebrows. (TV: The Day of the Doctor)

Clothing
The Curator wore a dark olive green blazer, a checked white shirt, mahogany red trousers and brown slip-on shoes with white socks. In his blazer breast pocket, he kept a red handkerchief. He also walked with a cane and wore a blue cravat under his shirt. (TV: The Day of the Doctor) He also once wore a black belt with a buckle and a jacket of a darker green. (COMIC: The Then and the Now)

Behind the scenes

 * The script for The Day of the Doctor never directly states that this character is the Doctor, but hints that he may be a future incarnation who has adopted the same form as the Fourth Doctor. The Then and the Now later depicts him as a future incarnation of the Doctor when the Doctor's timeline becomes distorted.
 * In the above scene he appears at the opposite end of the Doctor's lifespan to the First Doctor; this implies the Curator to be the final incarnation of the Doctor.
 * In an early draft of The Day of the Doctor, the Moment as portrayed by Billie Piper was the one to play the role of the Curator. Moffat considered that the Curator could still be interpreted as the Moment interacting with the Doctor using the Fourth Doctor's appearance, though Moffat personally preferred to interpret him as a very distant future incarnation of the Doctor who settled down and chose a different incarnation per day to revisit.
 * Tom Baker is not separately credited for this performance, receiving only a credit for playing "the Doctor" (as did all the other past Doctor actors, who appeared in the episode through the use of archive footage).
 * In DWM 487, showrunner Steven Moffat confirmed in the magazine's regular "Ask Steven" feature that the Curator is intended to be a future incarnation of the Doctor and suggests that, just as the War and Tenth Doctors retain no memory of the events of The Day of the Doctor, the Eleventh Doctor would not remember the specifics of his conversation with the Curator but will leave the Under-Gallery" with the strange, groundless conviction that Gallifrey is still out there". This was reflected in The Time of the Doctor in the Eleventh Doctor's uncertainty regarding whether Gallifrey was actually saved.
 * Tom Baker previously played several nameless characters suggested to be future incarnations of the Doctor.
 * In the 1992 release of Shada, Baker played the narrator for the unfinished links of the story. He described this as a "cancelled" adventure of his, and he mentioned that he had once defeated the Cybermen, Daleks, and Davros. He wore a pin stripe suit, a blue shirt, and a floral tie.
 * In 1997, Baker played the Doctor in a series of superannuation advertisements in New Zealand.
 * In an introduction for the 1999 Doctor Who Night, Introduction to the Night, Baker's future version of the Doctor referred to the Doctor in the third person but walked through in the Doctor's TARDIS and mentioned that he was "called Paul McGann in this one" when introducing TV: Doctor Who. He encountered multiple Daleks in the TARDIS before he introduced a "night of pure entertainment". This character was forgetful and consequently found himself laughing at most situations; he wore a black suit jacket, an open-neck shirt, and a white cravat.
 * Despite Big Finish's license being expanded to include the new series of Doctor Who, the BBC specifically instructed them not to use the Curator character in their audio dramas. (VOR 88)