Lemme just tell you what I think, flat out: Final Fantasy VI doesn't have a main character. Not at all, according to me, but certainly not in the way Final Fantasy has defined the term.
In Final Fantasy IV, Cecil Harvey was the destined hero of prophecy five ways till Tuesday. The story is all about the light and the dark, the vague ideals that live only in the hearts of man, and Cecil stands in the very center of that conflict. He never leaves the party. Final Fantasy VII is about control and its illusions, a looming terror from a distant star and the more wretched evil that greed itself has wraught. Cloud's psyche is like an optional dungeon of the worst kind, and you have to fight your way through it before he can land Omnislash on Sephiroth's beautiful bishounen face. Final Fantasy VIII is the story of how Squall and Rinoa became Squall-and-Rinoa, and how sweetness and light can change even Squall.
I could go on, but I don't think I need to. Final Fantasy VI is not Terra, it's not Locke and Celes, it's the when the grand finale starts playing and all the character motifs come together. I can't prove that, but I can talk about the three biggest players and why none of them is a true Final Fantasy main.
Terra is the most important character in Final Fantasy VI. At first, her mysterious power is a new hope for the rebel faction. As the nature of her powers is revealed, she's sought as an intermediary between the human and esper worlds. At the last hour, the story focuses on her liberation from uniqueness, as she stands at the head of the Falcon, hair unbound and waving in the breeze. Terra is the first character the player controls and the only one who will show up for the last battle if you fail to recruit her in the World of Ruin.
But for all this, the story moves around her, not through her. It quickly becomes apparent that the resistance isn't going to find a champion in Terra, but a half-willing tool. Terra's quest is to find her own humanity, not save the world, because you can't be a hero without knowing who you are. Commonly, if people aren't willing to give the game to Terra, they are at least willing to let her have the World of Balance. But Terra is absent from what might be the best plotted set of arcs in the game. From the Opera House to Vector, Locke becomes the player's steady point of view. And by the time Terra has come into her own, found love and hope and learned to clutch it to her chest and that place where it cannot die, her unique heritage and magical powers have ceased to matter.
Celes joins the party fairly late in the game-- she will be the fifth or the eighth character recruited, depending on what order you play the split-party scenes in. Once she joins, however, she becomes an important feature of the plot. Celes is interesting at first because of her questionable loyalties and strong personality. "I'm a general, not some opera floozy!" is one of the more famous lines in the game. When she leaves the party at Vector, her absence is dearly felt, and then, finally, she steals the show atop the floating continent. Here we have the traditional Squaresoft heroic angst, and it only continues on the Solitary Island, where Celes has to lose herself to find herself again. She becomes the anchor point for the second half of the game, and her search for friends and search for redemption intersect more than most would think.
But despite her being the first character the player sees in the World of Ruin, after her initial scenes are finished, Celes isn't any more important than any of the other characers you find and recruit. The World of Ruin isn't Celes' story-- Celes' story just takes place within the World of Ruin. She does help cheer up Edgar, and of course has something to say about Locke, but that's it, really.
The player wakes up three times in Final Fantasy VI: once in Narshe as Terra, once as Celes on the Solitary Island, and a third as Locke in Narshe again. Locke's the second character you're introduced to in the game and is a pretty considerable presence throughout the whole game. If someone's hurting, Locke will rush in to save the day, and isn't that what being a hero is about? Not to mention he's practically forced the whole way through the World of Balance. And if Celes fails to save Cid-- which she almost always does unless the player knows the trick of it-- Locke will be the reason she sets out from the Solitary Island, his bandana becoming a symbol of hope and life.
But the irony of Locke is that even though he's every inch the traditional hero, he's not the main in any way, shape, or form. His own tragic backstory has nothing to do with the plot unfolding except that he occasionally tries to blame the empire instead of himself. Kefka is trying desperately to reclaim Terra, he calls Edgar by name, but there's no indication he has any idea who Locke Cole is.
But that's sort of the point. The reason the game needs a nihilist villain like Kefka is because the real main character of the game is something like friendship, love, or teamwork. It can't exist without an ensemble cast. There's a reason the final showdown in Final Fantasy VI allows for a party of twelve, rather than a mano a mano duel to the death. No one can save the world alone. Terra needs Locke to rescue her in the beginning and Leo to educate her in the middle and the children in Mobliz in the end. She needs her parents courage to begin her existance. Celes needs Locke to rescue her in the beginning and Cid to nurse her back to health and the poetry of Locke's bandana and the hope of others' existance to go on and find the courage to climb Kefka's tower. Locke needs to save Terra and Celes and he needs Rachel to save him. Edgar and Sabin need each other and the promise of their kingdom. It's impossible to do a real single character playthrough of Final Fantasy VI. I'm not saying that Relm, Umaro, and Gogo are as important as Terra, Locke, and Celes. I am saying that if you force the role of lead onto someone you are doing the game a disservice.