In the allegory of The Lion, The Witch, and The Wardrobe, Aslan, the great lion, is described as being the Christ-character because he sacrifices himself, though he has committed no crime, to save the life of Edmund.
Why is he just mentioned as the Christ, though? I think it would be better to say he represents the whole Trinity - the Father, the Son, and the Holy Spirit - not just the Son.
Aslan is described as the King. Mr. Beaver says at one point, "'Course he isn't safe. But he's good. He's the King, I tell you." Aslan isn't safe, but he is good, just like my God. Aslan comes and goes in Narnia, and always at the right time. He does what he pleases, and what he does is good. Like my God, the thing he does is made good because he does it.
Of course, Aslan sacrifices his perfect life for a sinner, just like Christ. But there's more than that...Aslan was the active Creator of Narnia, just like my Jesus was the active Creator of everything which has been created.
Finally, Aslan is a comfort and a guide to his children, just as the Holy Spirit is to us. This is seen much more in the other books than in TLTWaTW, but it is still true.
Stay tuned for more ramblings on Narnia.