The Watcher (Logopolis)

The Watcher was a manifestation of the Doctor that began appearing in the hours prior to the regeneration of the the current incarnation Doctor into his next incarnation. He appeared to both the Doctor himself and his companions and brought Nyssa to the planet Logopolis.

Profile
The ghostly figure of the Watcher had a white, featureless, unformed face and body and white clothes. Though he would use the TARDIS later, he apparently had the power to traverse time and space without external aid.

He was first witnessed silently observing 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, as well. (DW: Logopolis)


 * It is not known for how long the Watcher had been observing the Doctor.

Although apparently silent, the Watcher was capable of communication, and engaged in discourse with the Doctor and, later, with his companion, Adric. The Watcher also, apparently on his own accord, transported Nyssa from Traken, a planet which would soon, unbeknownst to her, die due to the entropy wave, to Logopolis and safety. Later, apparently working on the Doctor's instructions, the Watcher piloted the TARDIS away from Logopolis to keep Adric, Nyssa and Tegan safe. It was during this voyage that the Watcher engaged Adric in a private conversation, the nature of which remains a mystery. (DW: Logopolis)

Ultimately, at the moment of the Fourth Doctor's death, the Watcher appeared once more and merged with the Doctor to create the Doctor's next incarnation. (DW: Logopolis, Castrovalva)


 * Exactly how and why the Watcher was created remains a mystery; to date no similar creature is known to have manifested in connection with any other of the Doctor's regenerations. However, a highly advanced Time Lord, K'anpo Rimpoche, had apparently manifested his future regeneration as his assistant, Cho Je, before regenerating into Cho Je. (DW: Planet of the Spiders)

While regenerating into his next incarnation, the Doctor once again saw the Watcher. (BFA: Circular Time)

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 onscreen, the novelisation suggest that Adric and Nyssa could hear him speak but possibly by telepathy. Both of them describe the voice as familiar while being unsure why they recognize it.
 * 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 "the moment has been prepared for." The knowledge passed to Adric is less certain, 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.
 * The Watcher- known as the Observer- of the Doctor's old friend Rallon appeared in 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.