>>11842097 (OP)As others have said, it's an ensemble cast for the most part, but there are some characters that are given the spotlight more than others, and those are Terra, Locke, and Celes.
For most of the first half of the game, Terra is literally the Girl Who is the Key to Everything, so most of the plot during that part revolves around her in some way, and even though she's technically optional in the second half, she still shows up to help in the ending, and she has the most complete character arc should you finish her recruitment quest.
Locke is the only character who is present for the entire first half of the game, only missing for the two scenarios he's not a part of early in the game, and his motivations and his past are a big part of the reason for that. He's the only character who is forced into your team for both the spots where that happens (the first being with Celes, the second with Terra), so unless you don't take him along to Zozo, the Sealed Cave, or the Flying Continent, he's there for it all. He becomes optional in the second half, but he serves as the major motivation for Celes to keep living and fighting.
As for Celes, she of course has the whole thing with Locke, but her place as a general of the Empire is kind of a big deal and figures a lot into everything that happens in the game from Vector onward, and proves decisive at the Floating Continent. She of course becomes the de facto main character in the second half, being one of the only mandatory characters, so that certainly helps.