He was really only an insurance policy, sad to say.
Impressing Barry Letts and Terrance Dicks from an appearance in Carnival of Monsters, Ian Marter was cast as the new companion for the fourth Doctor before Tom Baker was cast. It’s an old TV trope that when an actor either ages out in an action ‘pose’ or whose casting is unknown, to divide essentially one part into two, the physical and the intellectual. Around this same time Gareth Hunt (who appeared in Planet of the Spiders) was cast in The New Avengers opposite Patrick Macnee for the same reason. In both cases however it turned out the lead was more than capable of doing everything themselves and then the real question becomes what happens in story terms once this has been figured out. Tom Baker lamented during the making of Revenge of the Cybermen that Marter was being wasted for being given so little relatively to do, and he was right. With the one-off exception of The Android Invasion as part of UNIT’s last gasp, Harry Sullivan was a one season companion, something relatively unusual at that point in the classic series canon.
But what caught the attention of the Tour having recently re-watched blu-rays of Season 12 is a quirk Marter often had on screen. He stood there, often while observing the exposition of another actor in the scene, with both hands firmly on his hips. It happens often enough it becomes amusing to catch him literally in the act.
Harry Sullivan was infamously called an imbecile by the fourth Doctor, but he saved the Doctor’s life more than a few times during that singular season, and if he was specifically written for, as happened best in Genesis of the Daleks and Terror of the Zygons be rose above being a glorified cypher.
As for Ian Marter, his longer-lived contribution to Doctor Who came as an author during the heyday of the Target book range before an untimely death in 1986. The Tour has scraped (hard) to put together a Who Not Who! gallery for Marter, but it wasn’t easy. Most of his images were all related back to Doctor Who in one way or another!