Ferguson W. Darling (Jason Zimbler) – Clarissa's younger brother, a mischievous redhead. He and Clarissa continually antagonize each other. She refers to him with several epithets such as "Ferg-face", "Fergwad" or "Fergbreath."
He loves money and comes up with get-rich-quick schemes. He is also a Young Republican who idolizes Dan Quayle and Ronald Reagan. He goes to school with her and is about one or two years younger than her. Despite their rivalry, they occasionally collaborate, usually to the advantage of both. Unlike Clarissa, however, he does not seem to be very popular at school.
Jason Zimbler identified strongly with his character. He told Mashable that he's also "unrepentantly nerdy and proud of learning." He liked that Ferguson "made being bookish cool - or cool for being uncool."