VI has a lot of characters and often changes who you are playing as. As the game progresses there are fewer and fewer restrictions on which members are obligatory, and eventually you can make a part of whoever you see fit. This is in contrast to most other FFs and RPGs in general, where there's a central character who is always, or almost always, present, like Cloud or Squall.

The game also has a lot of character arcs and stories going on, and putting aside whether and how successful those are, most of the characters take center stage for at least a time. So it's a little harder than usual to pin down the protagonist. Is it Terra? You start as her and she plays a vital role, but she's absent for considerable portions of the game, and in the World of Ruin takes her time rejoining you. Celes doesn't show up for awhile and isn't fully trusted for a lot longer, but she's the one who reunites the party in the World of Ruin, and I think both need the other for their own arcs to be fully realized. Locke is vital at first, but takes a bit more of a back seat as time passes.

So, who would you consider the protagonist of this game? Deuteragonist? Tritagonist?