Harlan of Ash and Foam writes...

You answered this question recently,

http://s8.org/gargoyles/askgreg/search.php?qid=20324

And I'm here to respond to your question. I actually had NOT suspected Renee, and here's why (minor Spirits spoilers, I guess?):

It's not the first time for your work to feature important, lead characters who're advertised right up front to not actually appear until later: Artemis, after all. It essentially occurs again when Miranda features heavily in the synopsis for Rain of the Ghosts but is really a small, supporting piece of worldbuilding to pay off later. So it didn't seem odd to me that Renee would be introduced in, and become important during, book two while reasonably being a lead who was planned to feature from the very beginning.

Cash makes more immediate sense, and I'd at least suspected him. But my other guess was actually a small cheat: the three Kimlets as a singular unit.

The Kimlets themselves are very episodically important, but are spoken of in a way that feels pretty thematically important to the direction the series seems to be going. I'd wondered if you, often describing them in the book as a singular unit, had given them some more thought to take them even more in line with the (speculated) importance they may have to the wider series arc.