The "you" in When You Were Everything is a best friend, which is a trope I'm glad to see central in a novel. Normally I don't get a lot from epigraphs, but Shakespeare-loving Cleo's story is well-launched by the Antony and Cleopatra and MacBeth quotes about friendship and endings. 

The story is told in a before and after that was confusing to me because Cleo's love interest is similarly present in both, even though Dom's not a boyfriend in one timeline. Still, the two narratives help the reader understand how close Cleo's friendship with Layla was and how it broke. Layla is a singer, and once she gets accepted in the high school chorus, she begins hanging out with her fellow chorus girls more than with Cleo. The beginning of the dissolution is realistic and feels inevitable. Mistakes--cruel ones--are made by both girls. I appreciate that neither of them is clearly in the right. 

The secondary characters are believable--except maybe head bitch chorus girl Sloane, and the cast includes Black, Desi, and queer characters, and Layla is a stutterer. The friend groups are inclusive in a way that doesn't feel forced.