@Bookworm I thought about commenting on John Bollinger's answer, but I'm not sure it's sufficiently pertinent: I see some notable parallels between Rilian's fight with the Emerald Witch in serpent form and the Redcrosse Knight's fight with the monster Errour in The Faerie Queene, a text that Lewis would doubtless have known well. So it's possible the Emerald Witch represents not Satan per se but more specifically Error.

(There's no equivalent of the Silver Chair itself in The Faerie Queene, as far as I know, so any connections between the two works are not really pertinent to Rand al'Thor's question.)