The Demon Lord, by Nikki Frank

An imp living secretly among humans, all Olivia wants is a quiet senior year. The discovery that her only friend has ties to the magic world places him on the top of the Demon Lord’s list for collection. Now she’ll do whatever it takes to keep Emmett safe, even if it means working with a mini-menagerie of magic-folk who rub her the wrong way.

As the danger escalates, Olivia discovers friends aren’t nearly as hard to come by as she’d always thought. She soon realizes that friends are worth fighting for, and maybe even worth dying for.