Trevor Sandbourne is a Stoneybrook Middle School student in the same grade as the older members of the BSC. He has jet black hair, dark eyes, and freckles on his nose. He writes poetry for The Literary Voice, the creative journal at SMS.
In #2, Claudia and the Phantom Phone Calls, Claudia has an obsessive crush on Trevor, stalking him in the halls and daydreaming about getting lost with him on a field trip. She tries to ask him to the Halloween Hop, but accidentally drops a plate of Jell-O in his lap instead. The day before the dance, he calls and asks Claudia to go with him, and tells her that Alan had seen him writing a poem about the two of them once. Trevor also confesses to making some of the "phantom calls" to Claudia while trying to ask her out. (Also in this book, Trevor appears to be wholly unfamiliar with the "Sitting In A Tree" playground chant.)
Claudia and Trevor date informally in #3 and #4, but he isn't mentioned in book #5 and in #6, Kristy's Big Day, Claudia looks surprised that Kristy thinks she would go with Trevor to the Spring Fling and comments that Trevor is "probably dating his own poetry at this point. That’s all he cares about." (In #10, Kristy comments to Dawn that Claudia's previous giggly behavior about Trevor seemed to indicate that "she liked the idea of going out with him better than she liked Trevor himself.")
Trevor stops by the BSC's garage sale in #13, Good-bye, Stacey, Good-bye. He sits, at least sometimes, at the table of boys where Logan eats lunch when he doesn't sit with the BSC. He is in Mary Anne and Logan's "Modern Living" Short Takes class, in #52, Mary Anne + 2 Many Babies. He plays basketball with Logan and some other boys in #60.
In #57, Trevor and Woody Jefferson bring brie cheese, paté, and a bottle of sparkling cider to school for lunch, along with a tablecloth, silver and wine glasses, "to prove that we don’t have to eat Mystery Meat and pickled green beans to have a good lunch."
As seen at the Halloween Hop, Trevor is not a skilled dancer.