The Visitor was a Time Lord who was a member of the Celestial Intervention Agency. In 1788, he questioned Jamie McCrimmon about his involvement in the Glorious Revolution in 1688 after restoring his memories of his travels with the Second Doctor. He provided indirect assistance in stabilising and restoring the original timeline.
He later offered Jamie the opportunity to keep his memories. However, Jamie chose to once again forget his travels with the Doctor as he and his wife Kirsty had a good life together, which he did not think would have been possible if he remembered all that he had lost. (AUDIO: The Glorious Revolution)
Behind the scenes
The true name of this Time Lord is not known, so he is known as a visitor to Jamie.