Flying Spaghetti Monster wrote:
Right off the bat, I was thinking of the Fonze from Happy Days. I think he started as a very minor side-character. Soon he basically was the guy that was the ,mascot for the show.
Fonzie is probably the most prominent example. The other big ones would be Urkel from "Family Matters" and JJ on "Good Times" (which was originally supposed to be about the parents).
Also, now that I think about it, President Bartlett on "the West Wing" was originally supposed to be a supporting character, with Sam the main lead.