one

Alex has been able to see a number over people's heads, indicative of the destruction they're capable of causing, for as long as she can remember. Most people, including her best friend Lena, are between 5 and 15.
That was before grumpy, caffeine-obsessed, chemistry nerd Theo (whose number happens to be 20, the highest Alex has ever seen) transferred to her school.
Suddenly, Alex is on the run from zombies, caught in a love triangle, and trying to figure out how to balance her new superpowers with Theo's self-destructive tendencies and Lena's incessant optimism. When her city is threatened, will she remain the perfect student civilian, or step out of her comfort zone and rise to the challenge?

I own the cover art.