"Goth Girl Rising" by Barry Lyga

Time is a funny thing in the hospital. In the mental ward. You lose track of it easily. After six months in the Maryland Mental Health Unit, Kyra Sellers, a.k.a. Goth Girl, is going home. Unfortunately, she’s about to find out that while she was away, she lost track of more than time.

Kyra is back in black, feeling good, and ready to make up with the only person who’s ever appreciated her for who she really is.

But then she sees him. Fanboy. Transcended from everything he was into someone she barely recognizes.
And the anger and memories come rushing back.

There’s so much to do to people when you’re angry.
Kyra’s about to get very busy.