Basically Karen has to have her own way in everything, and if she is not the one who gets to be the center of attention when watch out, monster Karen is on the loose!
In this book Karen is upset that she wasn't the one who was gifted the pin, Kristy is, and she isn't allowed to borrow it. So of course she borrows it, then looses it, and somehow ends up with no punishments and Kristy ends up apologising to her!