Marguerite "Cokie" Mason is an eighth-grade student at Stoneybrook Middle School.
She is the chief rival and tormenter of the Baby-sitters Club. She is responsible for a number of pranks on the club, including sending ominous chain letters to spook Mary Anne in Mary Anne's Bad-Luck Mystery, and sending creepy love notes to Kristy in Kristy's Mystery Admirer. She has a crush on Logan and dated him briefly during Mary Anne and Logan's break-up (Mary Anne Misses Logan). She is a poor student and may or may not know how to read.
- Cokie's best friend is Grace Blume.