The Watcher (Logopolis)

During the moment that the Doctor's fourth incarnation regenerated into his fifth, The Watcher manifested as a physical projection. Eerie in appearance, the Watcher had a white, featureless, unform face and white clothes. He silently appeared observing events on Earth initially and conferred with the Doctor. Later, he transported Nyssa from the doomed Traken to Logopolis to safety.

At the moment of the Fourth Doctor's regeneration, the Watcher merged with the body. (DW: Logopolis)

Another Time Lord, K'anpo Rimpoche, had apparently manifested his future regeneration as his assistant, Cho Je. (DW: Planet of the Spiders)

In the novelisation, 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. The Watcher was never heard to speak in the television episodes but the novelisations suggest that Adric and Nyssa heard him speak but possibly telepathically. Both of them describe the voice as familiar while being unsure why they recognize it.