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).
Oprah: "Do the one thing you think you cannot do. Fail at it. Try again. Do better the second time. The only people who never tumble are those who never mount the high wire. This is your moment. Own it." - Oprah Winfrey -- I really admire her.