Who is the Lady of the Green Kirtle (Emerald Witch) and what relation, if any, does she have with Jadis?
In the book, the owls describe her as a Northern Witch and 'of the same crew' as Jadis. Perhaps, then, she was a follower of Jadis. Another suggestion is that the Emerald Witch is the White Witch in some form of purgatory. The message over the entrance to her realm speaks of the Lady as possibly being a ruler of the world above, as she is now a ruler below. This suggests after her destruction she somehow regenerated in the Underland.
Another option is that she's Jadis' sister. In The Magician's Nephew, Jadis battles her unnamed sister for control of Charn. Her sister is later presumed dead, but it's possible she survived and travelled to Narnia too.
There do seem to be other witches in Narnia, including the Hag who wishes to resurrect Jadis in Prince Caspian. It's also a possibility that the Lady is a former ally of the White Witch, who had served her as the Hag had; or that Jadis was wrongly affiliated with the Northern Witches (of which the Hag was a member), simply because she also had magical powers.
The old BBC series had Barbara Kellerman play the White Witch, the Hag and the Lady of the Green Kirtle, suggesting that all three were the same being in various forms. It is possible the Hag is the White Witch's debilitated form, and that by The Silver Chair she has had time to regenerate to a semblance of her former power. But this was only the BBC's interpretation, not Lewis'.
What do you think? Answers on a postcard please.