What is the theme in Confessions of an Ugly Stepsister by Gregory Maguire?

The idea of evil is a recurring theme brought to life by Iris's idea of an elusive imp that comes to mean different things as the story progresses. Iris believes she sees something evil lurking in the top floor of the van den Meer house when she moves in. It frightens her, but she is intrigued by it and seeks it despite her terror at what she may find. Since Clara is such an enigma at first, Iris wonders if she is evil, but the imp is not so easily located. It seems to loom intangibly like a bad omen over the household.