Shelby Marcus is the best friend of Cyd Ripley and one of the protagonists of Best Friends Whenever. She is Cyd Ripley's best friend and the two girls live together in Shelby's house, due to Cyd's parents leaving for Peru. Shelby and Cyd discover their power to time travel together by messing around with a science machine. Shelby is a girly, quirky, cheerful, and sassy young teen. She always likes to keep it safe, and is always struggling to keep her and Cyd out of trouble.