She and Polly begin their friendship when they play opposite one another in a school pantomime at age 12; Polly is Pierrot and Fiona is Pierret. Polly did not care for Fiona much before, but begins to like her during the wonderful performance which Thomas Lynn attends.
Fiona is imaginative -- she invents slodging -- but also sensible and sometimes tart, reminding Polly of Granny Whittacker. She regards the antics of Nina and Leslie Piper with some irony, and does not care for Seb.
Fiona, like Polly, goes to Oxford (although to a different college than Polly). In their second year, she and Polly become flatmates.