I agree with
Lady Courage and
Bookwyrm on this one in that I don't believe any one of Lewis's antagonists were meant to, on their own, be a seperate portrayal of Satan, but rather were representations or aspects of evil and Satan. Lewis points out in his Preface to the 1961 edition of Screwtape Letters,
The commonest question is whether I really "believe in the Devil".
Now, if by "the Devil" you mean a power opposite to God and, like God, self-existent from all eternity, the answer is certainly No. There is no uncreated being except God. God has no opposite. No being could attain "perfect badness" opposite to the perfect goodness of God...
The proper question is whether I believe in devils (emphasis added). I do. That is to say, I believe in angels, and I believe that some of these, by the abuse of their free will have become enemies to God and, as a corollary, to us.
Judgind by what he said, I think it extremely illogical to assume that Lewis would have ever tried to create one evil being that would completely and independantly represent a "Satan" of Narnia. I think it much more likely that through various characters he was showing different sides if evil and of the Devil and devils and how they affect us.
Like the oldest Dwarf says in The Silver Chair
"And the lesson of it all is, Your Highness...that these Northern Witches always mean the same thing, but in every age they have a different plan for getting it.
I take this to mean that there is not one great terrible evil, but rather a host of evils all sort of working together, with some more powerful and some less powerful, but all subject and less powerful than Aslan and always trying to work against His creation (if you look at it parallel to our world, then it means they are devils subject to and less powerful than Christ working against us). At least, that's how I see it.