SOLVED. Children/ YA book about two neighbors who are friends, both of them having gone through trauma. At the same time the girl is secretly the boy's bully. [s]

[DishwaterDan]

I read this book sometime in the mid to late 2000's, as early as 2003, as late as 2010 (sorry for the big range!). If I remember right the book was a little older, as most of the books I read at the time were secondhand. It was about a boy who lived with his grandma, or possibly his aunt. His mother had died in a fire (I think in an electrical fire on a Ferris wheel, or on another amusement park ride). The fire left him with big burn marks all over his chest. He doesn't have any friends and possibly has an imaginary friend. He may or may not be selectively mute.
The other main character is a girl who is obsessed with his tragedy. She is often a little manic and also struggles with mental illness (I can't remember if it's related to a specific trauma or if it's a general mental illness). She decides to befriend the boy, but also secretly sneaks out early in the morning and re-arranges rocks on the hillside by his house to spell out things she knows will hurt him. Stuff like "orphan" or "disfigured" or maybe even longer things. Every morning he wakes up and rushes out to scatter the rocks again before anyone sees what's written. Whenever she writes something on the hillside she copies down what she wrote on her stomach, writing it in ink and going over older ones to keep them visible. She uses her favorite pen (one of those pens with multiple colors of ink)
At one point the girl locks herself in the bathroom and colors in her teeth with a sharpie, noting that it makes them look like piano keys.
SPOILERS:
Somewhere close to the end they are both out in a rainstorm together. The girl's shirt gets drenched and the boy can see where she has written all the different things she wrote on the hillside on her stomach, realizing she was the one who had been writing them in the first place.
I think they make up in the end, I'm not sure though.
I have googled every permutation of those plot points. I am unsure of what watchlists I'm now on. Please help!

[Rainbowheart]

Words of Stone

[DishwaterDan]

Rainbowheart wrote: "Words of Stone"
This is it! Thank you!

[Rainbowheart]

Awesome, glad to help out!