Airborne Saga Series by Constance Sharper

Avery knew she had a knack for attracting trouble, but even she is shocked when a six-foot-something harpie shows up on her doorstep. Coping with the existence of a mythological race? Okay. Unwittingly finding herself in the middle of a vicious harpie conflict? A little less okay. Having to rely on an arrogant harpie boy who gets under her skin? Now that is something Avery isn't sure she can handle.

At last, Avery thought that she had made it back to a normal high school life-- except for the death threats on her head and the harpie in her backyard. Trouble never stays far away though, and Mikhail hatches a plan that endangers life as she knows it. Avery is drawn back into a vicious harpie war and is finally forced to confront her relationship with Mason whether she is ready to or not.

Two years ago, Avery never could have pictured willingly living with the supernatural civilization that had attempted to kill her. Not to mention joining that society when its monarch is an absolute upheaval. Naturally finding herself in the middle of it all, Avery is still fighting to stay in one piece. That is with or without the Willow magic.