Here's how Wikipedia describes the Doctor's companion Harry Sullivan:
Doctor Sullivan is a commissioned Surgeon-Lieutenant in the Royal Navy, who is attached as medical officer to the United Nations Intelligence Taskforce, the military organisation to which the Doctor acts as scientific advisor. He is first mentioned (though not seen) in "Planet of the Spiders", when the Brigadier thinks the Third Doctor has gone into a coma. The Brigadier calls "Doctor Sullivan" and asks him to come to the Doctor's laboratory, but tells him not to bother when Sergeant Benton wakes the Doctor by offering him a cup of coffee. In the next serial, "Robot", after the Doctor's third regeneration, Sullivan is called in to attend him, and ends up travelling aboard the TARDIS with the Fourth Doctor and Sarah Jane Smith (played by Elisabeth Sladen) for several subsequent adventures.
Harry is rather old-fashioned and stereotypically English in his attitudes. Somewhat accident-prone, he once claimed he was always trapping his nose in the doors of Portsmouth barracks. He often employs slightly archaic language — for example, referring to Sarah Jane affectionately as "old thing". He is nonetheless depicted as possessing great bravery and a "can-do" attitude, adapting well to the many strange situations in which he finds himself. He can, however, also be quite clumsy and unsubtle, leading the Doctor to once declare, in a moment of frustration, that "Harry Sullivan is an imbecile!" Nonetheless he is well liked by the Doctor and Sarah Jane, and has a slightly flirtatious relationship with the latter
The character was originally devised by the production team as a means of handling any action scenes required in episodes when they had envisioned that the new Doctor would be played by an older actor (Sarah Jane even jokingly compares Harry to James Bond at one point). When forty year-old Tom Baker was cast, however, this was no longer a concern and the decision was taken to write Harry out — something producer Philip Hinchcliffe later admitted was probably a mistake, as Harry was a likeable and popular character who worked well with both of his fellow leads.
Harry's last regular appearance is in the season thirteen opener "Terror of the Zygons", which had actually been made at the conclusion of the twelfth production block and held over to start the following season. At the conclusion of this story he chooses to return to London by train rather than by TARDIS with the Doctor and Sarah Jane, who continue their adventures without him. He does, however, reappear three stories later in "The Android Invasion", both as the original Harry and an android double. This is the character's final appearance in the programme.
A later production team gave some consideration to bringing Harry Sullivan back for a guest appearance in the 1983 story "Mawdryn Undead", part of the programme's twentieth anniversary season. Their first choice was the character of Ian Chesterton, but those plans fell through due to actor William Russell being unavailable. In the end, they decided to use the character of Brigadier Lethbridge-Stewart (played by Nicholas Courtney) instead. Harry is mentioned in the story, however — the Brigadier tells the Fifth Doctor that he was "seconded to NATO" and was last heard of "doing something 'hush-hush' at Porton Down."
Just based on that picture above, I'd like to suggest that Harry had a mentor in learning about doing things 'Hush-Hush'.....
And it wouldn't have been the first time that John Steed, top professional, worked with a talented amateur who was also a doctor. When we first met Steed, he was teamed up with Dr. David Keel.....