Eilonwy

From what I've been able to tell, she's about twelve in The Book of Three (she's described as being a couple of years younger than Taran). And I don't suppose that you could put a certain oracular white pig in the drawing instead of the Horned King? ;)

Here's the description of her from her first appearance in the book:

"Eilonwy, as the bewildered Taran saw her for the first time, had, in addition to blue eyes, long hair of reddish gold reaching to her waist. Her face, though smudged, was delicate, elfin, with high cheekbones. Her short, white robe, mud-stained, was girdled with silver links. A crescent moon of silver hung from a fine chain around her neck. She was one or two years younger than he, but fully as tall. "