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. They become good friends the following term.
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 amusement tinged by irony, and does not care for Seb. Although she matures into one of the prettiest girls in the school, she does not seem to let it go to her head.
Her 15th year is a memorable one for Fiona. During the summer, she gets chicken-pox, followed by shingles, and is quite ill for over a month. Later that year she astonishes everyone by falling madly in love with a German businessman named Hans whom "her father happened to bring home one evening" and runs off to Germany in pursuit of him. (He sends her home with a kindly pat on the head. Apparently she kicks him in outrage at this dismissal.)
Fiona, like Polly, goes to Oxford (although to a different college than Polly). In their second year, she and Polly become flatmates.