I don't know about Richard Hunt's first main SS character, but I just wanted to say that, as an '80s kid, until I recently saw some '70s episodes for the first time, I had no idea some of the characters had been around as long as they have. Gladys, in her familiar form, has been around at least since 1975. And Little Chrissy dates from at least 1974, which really surprised me. For some reason I'd always assumed he was an '80s creation.
These two are not among the characters you typically see used in promotional stuff, featured on merchandise, and in all the storybooks and stuff, so it's somewhat surprising to realize that, despite their second-banana status, they've been there a very long time, faithfully toiling away in the background.
Regarding Hunt, if indeed Sully has been around since 1973, I would wager that he was Hunt's first important character.