Demi is nice as long as you dont cross her. She was picked on when she first came to the Treehouse by a particular group because she mysteriously arrived in Neverland (aka through a wormhole), she wasn't picked by Pan so that caused some resentment thinking she had a charmed life. It's since changed since she keeps losing those closest to her. Still, she's fiercely protective of the Treehouse because they're all she's got.
Lys isn't nearly as complicated. After all, fae can only really experience one emotion at a time. If she gets mad, she'll probably throw a short tantrum and then be back to her adorable self. She LOVES flying, and shiny things.