Harry was an interesting character-originally, he was intended to be an Ian type to the Fourth Doctor, who originally was going to be a more Hartnellesque Doctor before they cast Tom Baker.
Despite appearing in only a few stories, he was kind of IMO a prototype for Rory, a well-intentioned, intelligent and capable physician, but a little clumsy in the way of adventuring at times. He had good chemistry with Baker and Sladen too IMO.
BTW speaking of that, it's sad that we've lost some of the Baker era companion actors to cancer. Sladen, Marter, Tamm.