Sidekick-o-rama : Harry Sullivan (companion to the 4th Doctor)

Harry Sullivan (Ian Marter)

Poor Harry seems to have faded into obscurity.

He accompanied Sarah Jane Smith and Tom Baker's Doctor for a season and, well, it's easy to be eclipsed by those two.
In many ways, he was the prototype of current companion Rory.

It's hard for writers to create memorable male companions who don't detract from the Doctor. (I can only think of moody Turlough and, arguably, Captain Jack).
Harry's purpose was to be the man of action as the producers had wanted to go for an older Doctor. This was made rather pointless when they cast Tom Baker.
So Harry evolved to be sweet albeit a bit hopeless. Witness this lovely scene from Revenge of The Cybermen.
Harry was a Navy medic and acted as a nice counterpoint as the show switched from an establishment-friendly Third Doctor often working with the military to the iconoclastic Fourth.
His old-fashioned decency stands out. Yeah, he's a figure of fun sometimes but he does the right thing.
And his unrequited unspoken love for Sarah Jane is rather sweet looking back.

He's a bit of a third wheel and who hasn't been there?
And his run managed to include all-time classic Genesis Of The Daleks.
Also, a few years later Ian Marter, the actor who played him, wrote a spin-off novel in which Harry found himself embroiled in Van Gogh-related espionage.

My old French exercise book suggests that I regarded this as the best book I'd ever read. To be honest, this might not still be the case.
But few other companions would come close to inspiring something like this.