Chaos (Kardia Chronicles) by Christine O'Neil
My name is Maggie Raynard. After sixteen years being just plain me, suddenly, when I lose my temper, my fingers become weapons of mass destruction. Turns out I'm a semi-god, descended from Aphrodite. Sounds cool in theory, but when I accidentally put my ex-boyfriend in a coma, things go downhill pretty fast.
Now some new guy named Mac Finnegan has made it his mission in life to continually piss me off. I'm stuck learning how to use my new powers while also dealing with regular high school problems, and with this annoying—and super-hot—guy all up in my business, I'm about to flip out.
But it gets worse. I just learned there's this watchdog council of semis who keeps an eye out for any bad apples. They think I'm the baddest of the bunch and want to take me out before I do any more damage. My nemesis Mac might turn out to be my salvation, only he's got secrets of his own...and they may just kill us both.