Well, without Aslan there wouldn't BE any Narnian stories, or none worth getting excited about. Other than Aslan, I think most would consider Lucy to be pivotal, since she has the most open heart to Aslan.
Did I ask a question like this? Can't remember, but I'm glad it's getting attention. I know Aslan is the main character for all of these stories, but I think that Digory is the key character. After all, he did ring the bell...
Aslan, of course, then the Pevensies. Lucy especially, then Edmund. Lucy since she kind of "started it all" in a sense, and Edmund for being the example of what Aslan sacrificed Himself for.
I didn't mean it like that! I think that you are thinking of the catylists is the story... the people who make it more interesting. I still stand by Digory and Peter because they sort of established Narnia.