It's similar to the Lord of the Rings. You could make a case for Frodo, Aragorn and Gandalf being the main character (and although a lot of people would say Frodo, I'd disagree) and it's the same with FFVI. Terra, Locke, Celes, and possibly even Edgar have claims to be the main character.

I'm not trying to say "There is no main character", but rather, "There are three or four main characters".