who is harry sullivan?

Harry Sullivan, played by Ian Marter, is a character in the British sci-fi show Doctor Who, and was one of the 4th Doctor's companions. He is a surgeon-lieutenant in the Royal Navy, and a medical officer at UNIT. He is rather old-fashioned and very clumsy (causing the Doctor to proclaim very loudly that he is an imbecile), but still brave and strong and adapts well to the strange situations he finds himself in. He was introduced in the episode Robot, in 1974, which was also the 4th Doctor's first episode. He was told by Brigadier Lethbridge-Stewart to take care of the Doctor post-regeneration, which lead to him getting tied up in a cupboard, and shortly after joining the Doctor as a companion.

As all of the previous actors of the Doctor were older, it was assumed that the next Doctor would also be older, so Harry Sullivan was written to handle action scenes, although when Tom Baker was cast at 40 (the youngest actor of the Doctor up to that point), Harry was no longer needed, and was written out after only 6 stories, although he appeared in a final 7th not long later.