No. 33 (Enlightenment)

In Earth-33⅓, "No. 33" was the caption of a framed picture of a male humanoid, which an exhausted Peri Brown once glimpsed while climbing a punishingly long staircase inside the Doctor's TARDIS, alongside several others like it.

"No. 33" was a humanoid with an extremely enlarged, bald skull, his forehead emblazoned with a question mark. He appeared calm on the picture, through he had darkened, heavy eyebags. (COMIC: Enlightenment)

Behind the scenes

 * As portraits labeled "No. 1" and "No. 2", appearing in the background of the same story, are clearly depictions of the First and Second Doctors, readers are led to the assumption that the other individuals in numbered portraits are other incarnations of the Doctor. In the cases of both "No. 33" and "No. 13", this reading would imply they are in fact depictions of future incarnations of the Doctor relative to the Sixth Doctor (who stars in Enlightenment). TV: The Doctor's Wife and Journey to the Centre of the TARDIS would later elaborate on the idea of potential futures leaking into the "present" of the timeship's interior — although it may be simpler to assume that the picture does not in fact depict the Doctor, or, even if it does, that it is a fabrication of some sort.
 * "No. 33"'s enlarged forehead grant him a certain resemblance to such characters as the Mekon or the brain child of Uranus.