It depends on how you define "main characters". The "mainest" were Jill and Eustace, who are brought from our world to Narnia to help King Tirian. They stay with him through the Last Battle itself and are thrown through the Stable Door. However, the others (Digory, Polly, Peter, Edmund, and Lucy) show up in Tirian's dream early in the story - which is actually him appearing to them at a dinner as a phantom-like vision - and at the end, when the truth becomes known about this visit to Narnia, and how it differs from the others. In a sense, everyone but Jill and Eustace have bit parts, but in a sense, it's their greatest story.
Have you read the book yet?