Foil
Character Role Analysis
Seth and Kendra
Seth is almost up there with Kendra as a protagonist, but we stuck him in the foil category because he's not quite the hero of the story in the same way that she is—though like his sister, Seth's not a bad guy either (though the fairies might beg to differ). Instead, his presence highlights the differences between Kendra's choices and his choices, and helps us see what a winner Kendra is.
Seth's the one who messes up everything at Midsummer Eve by peeking out and eventually opening the window, letting in the monsters. And in a way, he pre-messed-up by angering the fairies beforehand, so their presence wouldn't help protect the house during Midsummer Eve. Did Kendra do any of that stuff? Nope, she behaved and followed the rules. Seth and Kendra are like night and day, and that's why we're saying Seth is the rebellious foil to Kendra's good-natured obedience.