Fern Halpin is a fifth-year student at the Legatum Continuatum School for the Descendants of Famous Detectives. She is Ivy Halpin's sister and best friends with Lucky Ng, Clive Ng's sister. Her boyfriend is Salty Pinchbeck, an emergency medical technician. She and Ivy look a lot alike and are both tiny.

Fern specializes in textual analysis and language and often works with Professor Bill Pickle. She knows everything about the school and Ivy often relies on her for critical information. She first appears in Book 1 of the Amanda Lester series, Amanda Lester and the Pink Sugar Conspiracy.