This mysterious figure has a shrine on an island in Fablehaven. All Kendra really knows about her is that she's immensely powerful, and the last time a human set foot on the island, he was turned into dandelion puff. Oops.
Her physical manifestation at the shrine is as a "two-inch-tall statue of a fairy, finely carved. It rested on a white pedestal that added a few more inches to the height" (17.76). When Kendra narrates her story and her plea for help, "a presence" (17.87) appears and speaks in Kendra's mind, accepting her offering and promising aid. And… that's all we really see/hear of the Fairy Queen. Mysterious much? Yep.