Actually, “MacGuyver” also had his boss, Pete (don’t recall the last name, but he was played by Dana Elcar–IMDB lists the character as “Andy Colson” but I remember it as “Pete”) in just about every episode. Plus there were several recurring guest roles that bordered on regulars.
Didn’t the guy tracking the fugitive appear in most episodes of that show? Was he in the opening credits? That would make him a regular.
For some reason, the first one I thought of was “The Incredible Hulk”, but that had two characters, David Banner and Jack McGee (another fugitive and tracker pair).
“Quantum Leap” was definitely two characters. Gushi appeared a couple of times, though he was named in nearly every ep, and Ziggy’s voice was only heard a couple of times per season (though Al spoke to ‘her’ a lot, we couldn’t hear the replies most of the time).
How about “The Twilight Zone”? It only had its presenter on regularly, so while I guess Rod Serling (or Forest Witaker) counts as cast, he was on screen for less time than almost any regular character in history. Or “The Outer Limits”? Zero regulars (unless you count the Control Voice, but that was really just the credits announcer–If you count it, you need to count theme-song singers). But I guess those don’t count, having no series continuity.