I need to figure out who killed the king.

OK, there’s a bit more to it than that. I’m in the will-this-shiny-new-idea-work stage of a new novel. It’s coming together beautifully and I’m in love with the main characters. But I’m still trying to fill in pieces of the overall plot to make sure it works. It’s a high fantasy sort of setting. Here’s the basic premise and where I’m running into trouble.

Basic premise as it applies to current problem: The King is assassinated. His teenage stepson walks in on it and sees it done. Stepson escapes that night and is missing for ten years. He comes back. Conflict that’s been raised/resolved/brewing/etc over all this time comes to a head/ensues.

The plot turn where I’m having trouble: One of the ways the shit hits the fan with this return is that it was never determined all this time who actually killed the king. There have been powerful dukes and the king’s younger brother implicated in the overall plot, but no one knows who wielded the knife, or if they do, they’re not telling. But the stepson saw who it was all those years ago. So now that he’s back, he can out the killer.

Problem: I need a character for this killer. Thing is, there’s an alternate world attached to this one where each person has a counterpart, though, as often is in one of these stories, ones who are good in one world are evil in the other, etc. I have a counterpart for this character in the alternate world that I need to match up to someone in this one. For that reason, I need this person to be:

A woman.

Someone both the king and the stepson would know.

It doesn’t have to be someone they liked or trusted but there’s no reason it couldn’t have been, either. It would probably fit better with the alternate world if this one wasn’t clearly and obviously a friend of the king.

Someone who is now safely out of the way, her compatriots probably know where and have means to contact her. She may or may not have disappeared as soon as the deed was done, but she’s not in reach now that the stepson is back at the palace to name her.

She’s someone no one suspects. (This one makes me leery of using wives/sisters of any of the known plotters.)

Limitations: The boys’ mother was already dead, the king had no other lover. They have no other close female relations, except possibly a wife for the king’s brother, a known plotter.

I’ve got no major female character already in this part of the story at all–it’s one of those icky patriarchal societies–;) except one who is too young to have been involved. I don’t have a problem inserting one, and she doesn’t have to play a hugely important role otherwise, but does have to be enough a part of the backstory¬†that it’s not a “who?” moment when she’s revealed. (Obviously she can’t play a huge role now, since she’s not around to be captured once the prince names her.)

Anyone have any ideas? Or have I backed myself into a corner and need to revisit some of my limitations?