I just got this game yesterday. And in the process of doing the Origins leveling cheat/trick thing, I raised Firion and Maria's Sword and Bow skills (respectively) to level 4, and then tried to level up their hp. They "one-hit" (it was more like 4 or 5 hits :P) KO'd themselves, and, being the n00b I am ;x, I didn't know you could revive them at the altar.

So I dinked around as Gus, taught him Cure, and against the weak monsters outside of town rose his MP, HP, and weapon skill by attacking himself until he was really low and curing back to full and repeating. Now he has 180ish HP and 23ish MP, while Firion is 30/5, and Maria is 20/11.

This makes me wonder, though.

would it be possible to go through the entire game just using one character?

Yes it would, and it has been done.