What is really wrong with Z?

It’s hard for me to answer this question fully without some spoilers, but I’ll try. Z lives in a fantasy world, which Ella accepts about him, but much of his behavior in the book is quite strange. Readers speculate that Z might be affected by Asperger’s Syndrome, or autism, or one of any number of emotional or psychological disorders. Maybe. But at least part of what affects Z is a form of post-traumatic stress, caused by the sudden and difficult changes to his family and home life. He protects himself from the harsh realities he faces by spinning fantasies that make him feel good. I chose not to “diagnose” Z in the course of the book, mainly because the story is told from Ella’s perspective. She would not necessarily know what teachers, administrators and doctors might be saying about Z. Ella accepts Z’s differences just like he accepts hers.