One property of a main character is to stick with your party like glue. No character have that property.

If I were to say, then Locke would be the main character as far as WoB goes. The world revolves Celes too...lightly =/.

In WoR, I could be suggested the Celes be the main character but that was only first section of the WoR. Once you get Sabin, the main character property of her slowly dissipates.