Robert McIntosh

 was the Fifth Doctor's assistant for over about a year while the Doctor lived as "Dr Walters" at 107 Baker Street in London during 1866 and 1867. A medical student, he was unaware that the Doctor was anything other than a human academic until after Nyssa returned to the Doctor in late 1867. When he realised that he didn't know the Doctor at all, he was furious with the Time Lord and immediately proposed to end their friendship. Ultimately, however, McIntosh sacrificed his life to save the Doctor and Nyssa. (AUDIO: The Haunting of Thomas Brewster)

Behind the scenes
McIntosh is an unusual companion because we only see his final adventure with the Doctor, and it's only in this adventure that the Doctor reveals something of his true nature to McIntosh. However, he has been in apparently daily service to the Doctor, and is called both an "assistant" and "good friend" by the Doctor.