Hm.. I'll go with the general consensus and throw in Gordon from Final Fantasy II and Edward from Final Fantasy IV.

That being said, I've never really had much use for Relm and Umaro in Final Fantasy VI. Relm is useful, yes, but I've always been bothered by the fact that she has better magic than Terra (who's supposed to be half-magical being herself). That, and her glitchy special ability, sketch, put me off. And while I like Umaro (especially when equipped with the Rage Ring), I never used him much because I don't like not being able to control him.