I actually really like Emma Stone, but being a redhead (or at least often-dyed-red-head) with a crazy gleam in her eye, she'd do a good job as the psychotically deadly Krystal who joins Thanatos in book 2 (Underworld's Daughter).
She was beautiful, but not like those girls in the magazines. She was beautiful for the way she thought. She was beautiful for that sparkle in her eyes when she talked about something she loved. She was beautiful for her ability to make other people smile