The Youthful Doctor
This Doctor is definitely an agent of C19. We know this because he used their name and was backed by Sir John Sudbury during the Heathrow Incident. He was involved in a number of incidents in the early 1980s, and may well have been a successor to the Bohemian Doctor. Although he only actually called on C-19 resources once, he was very clearly connected to them. The most notable incident he was involved in was the Brendon School Affair, which also involved Brigadier Lethbridge-Stewart. More recently, this Doctor was involved in the Manchester Incident, which occurred during 1993, although descriptions of him then place him in the same age bracket as his description in the 1980s.
The Youthful Doctor appeared to be in his late 20s, with straight fair hair. He always wore a cream top coat and stripy trousers with a cricketing pullover, white cricket shoes and a hat. For some unknown reason he wore a sprig of celery in his lapel. The most likely explanation of this is that it was some sort of communication or homing device, although it could have been there for some other reason or merely as a spot of eccentricity. He always came across as a heroic and vulnerable character, putting himself at risk rather than others in dangerous situations during incidents he was involved in. He was quiet and tended not to command situations very strongly. He may well have been less experienced than some of the other Doctor agents due to his relative youth, an ample reason for his lack of control of situations in contrast to other Doctor agents, although that may well have been a ploy to make people underestimate him.