Character Analysis
While Jill seems to make the connection between the serpent that kills Rilian's mother and the lady in green who enchants Rilian and abducts him, neither she nor Eustace seem to think of her when they meet the beautiful lady in green who just happens to be out for a ride on the decaying giants' bridge.
While it's possible that the Lady is just a mistress of disguise, it's more probable that Lewis was making a statement about how desire can bewitch the senses. Jill is quite taken in by the beauty of this woman and her presentation, while Puddleglum more sensibly feels that something fishy is going on. Jill and Eustace succumb to the everyday enchantments of a good appearance and a posh voice—and the possibility of a bed and a hot bath. Eustace thinks she's "simply super" and doesn't stop to consider that her green costume and convenient arrival should raise some red flags.
The Queen of Pain
The Lady's transformative powers make her difficult to pin down and define. One moment she's a beautiful lady with a silent knight, the next she's the Queen of Underland—and then she's a deadly serpent. We really can't blame the children for trusting in her beauty and smoothness when she appears to them on the giants' bridge—after all, it's not until they've listened to the Black Knight's story, witnessed his transformation back into Rilian, and encountered her as the Queen of Underland that they get a proper view of her character.
And what a view it is. If King Caspian is a model king, the Queen of Underland wrote the book on wickedness. The Black Knight reveals that she has plans to overtake a kingdom in the upper world, slay its king (a.k.a. Narnia and Caspian, respectively), and subjugate its people. While the enchanted Knight thinks this is a great plan, we know how twisted the story has become: The Queen is going to make Rilian kill his own father and hand over a crown that is rightfully his. Not cool, lady, not cool.
She's also in the business of magic and mind control. Of course, the Black Knight/Rilian has been enchanted and believes that the Queen is saving him from some wicked curse. But that's just the start of it all. Once the crew escapes her enchantment and she is killed, they notice that something peculiar is happening to the land around them: It begins to fall apart.
Puddleglum realizes that the "Queen" was not really a ruler, but a witch who brought things under control through magic: "That Witch has laid a train of magic spells so that whenever she was killed, at that same moment her whole kingdom would fall to pieces" (13.189), he says. After they meet Golg and hear the story of how she enslaved the race of gnomes from Bism, they finally have a pretty good sense of whom they were really dealing with.
Can We See Your I.D., Ma'am?
So who is this murderous person (if person is even the word)? The Black Knight gives an important clue when he tells the story of the words "Under Me" that are carved on the top of the ruined giant city:
Those words meant nothing to your purpose. Had you but asked my Lady, she could have given you better counsel. For those words are all that is left of a longer script, which in ancient times, as she well remembers, expressed this verse […]. (10.153)
The devil is literally in the details of this passage, as Puddleglum well knows, because he picks up the phrase "in ancient times, as she well remembers" and hangs on to it. He remarks to the Black Knight that his lady must be a "long liver" if she remembers the verse in its original. The Black Knight tells them that indeed, "She is of divine race and knows neither age nor death" (10.154). Doesn't sound like an ordinary human to us…
When the crew surfaces in Narnia and Rilian has the opportunity to consult with the oldest and wisest dwarves about his adventures, they confirm the original identification of "Northern Witch" and remind us that they "always mean the same thing, but in every age they have a different plan for getting it" (15.227). Happily, like the White Witch before her, the Queen of Underland and her evil cannot overcome the powerful good of Narnia.
The Lady of the Green Kirtle / The Queen of Underland's Timeline