You know stories have or can have more than one protagonist, right? I mean, the entire cast are protagonists.
But to play along:
Is there any character who can't be removed from the story without it falling apart?
Terra is a bridge between many of the characters and plot points of the story. She is literally the bridge between humans and espers. The opening scene involves Terra, and thus she gets the ball rolling. If you take her out, then what happens? The emperor sends someone else in Terra's place. That person dies along with Biggs and Wedge upon confronting Tritoch, Locke doesn't have anyone to rescue, the moogles aren't needed, Edgar isn't inspired to finally openly turn on the empire, celes never gets rescued, sabin descends into madness, the espers lose their trout and kill everyone, etc., etc.
However, that just means she's crucial to the plot. And being crucial to the plot does not make a character the main character. And I think it's about more than just screen time, too.
If you had to sum FFVI up into a single word or phrase what would you use? Community? Redemption? Willpower? Love? Justice vs. Peace? Technology vs. Magic? Being oneself? Balance?
I'm not trying to be objective about this, mind you. This is how I, personally, am choosing to evaluate who I think the primary character might be.
Mog
Umaro
Gogo
Relm-- No redemption, not about willpower.
Strago-- Fighting because of Relm, not about balance or will
Shadow-- Though he joins the group in the end, he finds no balance, and he still rejects community. His redemption is limited.
Gau-- Gau actually fits several of FFVI's themes, but falls short in the end because in accepting himself, he's accepting only what the world made him, not what he chose to be. As in, he never truly moves beyond the wild child who belongs on the Veldt.
Edgar-- Edgar is Edgar. The end.
Sabin-- Sabin is a strongish character who develops quite a bit and fits many of the themes, but he too falls short by lacking several of the key components. He doesn't deal with technology or magic, for one thing.
Setzer-- Setzer might seem like a background character to many, but he's one of my favorites and he's one that I think represents FFVI rather well. I just don't think the story digs at his character enough to uncover everything that would be necessary to see Setzer as THE main character. A lot of his story is only hinted at or mentioned in passing. He's still a bad-ass, though.
Cyan-- Cyan comes very close to being the best representative of FFVI's overall thematic. He deals with all the themes listed above and then some. But in the end, he does not exceed his limits. He merely recognizes his role in the community post-tragedy.
Locke-- Locke is definitely one of the main characters. He practically disappears in the second half of the game, though. He's typically going to be one of the last cast members you get back in the WoR, and even though I enjoy his second-half story, it seems to take a step backward instead of a step forward. It's more back-story than it is this story, and thus he can't be my main character. He's still up there, for me, up in the top 3-5. Typically when I think FFVI, I think Locke, Celes, Terra.
Celes -- Celes fits every theme. Her game-story encompasses everything FFVI is trying to say about technology & magic (she's a magitek knight, for smurf's sake), about willpower, about corruption, about love, about redemption, about everything. And finally, I didn't list it above, but one of the central themes of FFVI is hope. It's probably the primary theme of the WoR. And it starts with Celes. With the desperate attempt to save grandpa, with the battle against ending her own life, and with the decision to strike off into the world on her own to find everyone.
Terra -- Terra's story is about being who you were born to be, about fighting for community, about the dangers of both technology and magic, about the balance between willpower and anger, justice and peace, about love and loss. And like Celes before, Terra's story is about hope. In the end, Terra accepts both her humanity and the beast inside her and brings balance to the force. I mean, brings balance to what's left of humanity in the WoR. I won't get all crazy into this trout, but the big ugly green thing that you have to fight multiple times in the Terra/Mobliz quests is Humbaba, a giant in ancient mythologies who guarded a forest where the gods dwell. So in killing him, Terra is accessing godhood. This is meant to be a character contrast to Kefka. They both achieve godhood, but through different means and for different reasons. (Terra through humility and hope, Kefka through avarice, pride, and malice).
One way to determine the true main character would be to find the character who plays a foil to the primary antagonist, and Terra is exactly that to Kefka.
But so, in many ways, is Celes. (both magitek knights, etc.)
They're both protagonists. And not just one is the protagonist of the first half and the other is the protagonist of the second half. They each carry their stories through both halves of the game. They're dualgonists. Or something.