I have to argue that Terra is the main character. Her theme plays when roaming the overworld. You start with her and she stays until (SPOILER)the split scenarios. I've also heard she joins you at the end whether you get her or not. She also triggers much of the story in the first half. Celes could take over in the third quarter, but near the end, you can use anyone. So, Celes is it in the (SPOILER)Opera House and start of the second half. Terra has most of the first half.