The Watcher (Logopolis)

The Watcher was a manifestation of the Doctor who appeared during the events that led up to the regeneration of his fourth incarnation into his fifth. He appeared to the Doctor and his companions, and brought Nyssa to the planet Logopolis. He appeared to be the interim period of the Doctor's fourth regeneration, between the fourth incarnation's death and fifth incarnation's “birth”.

Appearances
In a dream, the Fourth Doctor saw the Watcher and thought of his next incarnation. (PROSE: Into the Silent Land)

The Watcher silently observed events on Earth, particularly the events leading to Tegan Jovanka entering the Doctor's TARDIS. It was around this time that the Doctor first noticed the Watcher. Although apparently silent, the Watcher was capable of communication. He communicated with the Fourth Doctor, and was seen apparently carrying on a conversation of some sorts, though what was said was unknown. Later, he communicated with the Doctor's companion Adric. The Watcher, apparently on his own accord, transported Nyssa from her home planet of Traken, a planet which would soon die in the entropy wave, to Logopolis and safety. Later, apparently working on the Doctor's instructions, he piloted the TARDIS from Logopolis to keep Adric, Nyssa and Tegan safe. It was during this voyage that the Watcher engaged Adric in a private conversation. As the Fourth Doctor died, the Watcher appeared and merged with the Doctor to create the Doctor's next incarnation. (TV: Logopolis)

During his own regeneration, the Fifth Doctor again saw and merged with a recreation of the Watcher that reflected the Sixth Doctor in his mindscape. (TV: The Caves of Androzani, AUDIO: Winter)

Shortly before his regeneration, the Tenth Doctor saw a solitary pale watcher observing him; shortly after the TARDIS dematerialized, the watcher also disappeared. (PROSE: The Advent of Fear)

Description
The Watcher was a ghostly figure with a white, featureless, unformed face and body. He wore white clothes. Though he used the Doctor's TARDIS at one point, he apparently had the power to traverse time and space on his own. (TV: Logopolis)

Behind the scenes

 * In the novelisation of Logopolis, the fact that the Watcher was the Doctor was indirectly alluded to during the TARDIS scenes where Nyssa and Adric were being taken away from Logopolis by the Watcher, allegedly at the Doctor's request. While the Watcher was never heard to speak on-screen, the novelisation suggests that Adric and Nyssa could hear him, possibly by telepathy. Both of them find the voice familiar, but are unsure why they recognise it. When Adric tries to remember the Watcher's voice, he finds he can only recall the Doctor speaking.
 * The nature of the Watcher's conversations with the Doctor and Adric was never revealed. Logopolis suggests that the Doctor was made aware of his impending regeneration due to his statement "It's the end, but the moment has been prepared for." The knowledge passed to Adric is never revealed, although after the transformation, Adric does show an unexpected knowledge of regeneration in Castrovalva.
 * Several other "partial Doctors" have appeared in the history of the series, most notably an evil manifestation, the Valeyard (TV: The Trial of a Time Lord) which at some point the Sixth Doctor beleived to be some sort of Watcher although others remarked that he did not look like one (AUDIO: Trial of the Valeyard).
 * The Watcher of the Doctor's old friend Rallon appeared in the novel Divided Loyalties, merging with the Celestial Toymaker — who had possessed Rallon's body for his own – after Rallon's death to keep the Toymaker in check in future, the transition from Rallon to his Watcher causing the Toymaker's personality to change as though he himself had regenerated. This Watcher was known as the Observer.
 * A figure like the Watcher appeared in the audio story Renaissance. After Romana returned to Gallifrey from the Axis, the Watcher was seen lingering around the Capitol, shortly before Romana's regeneration after her fatal exposure to the Eye of Harmony. However, it was later revealed in the audio story Ascension that this occurred in the Matrix, and her regeneration was negated when she was extracted from the Matrix.