My, my. It's sad that some people are completely unaware of the fact that FFV and FFVI were ported from the SENS, originally FFII and FIII. Sorry, back to the topic.
I believe that if this game any main character it would have to be Terra. I know that this game tries to have no real central character to this game, but Terra is a close as you can get. Namely the fact you start off with her and that her story is more integral to the plot of the game itself say as much.
Sure, you could say Celes was the main character after the Kefka incident but that doesn't stop Terra from participating in the final battle whether you pick her up or not.
