She is the only child of Reg and Ivy Whittacker, who neglect her despite the fact that she is pretty, clever, imaginative, and well-behaved. For much of the book she lives with her grandmother. In school her close friends are Nina Carrington and Fiona Perks, as well as Leslie Piper; gossip has it that Seb Leroy is in love with her.
She has long, very fair hair, and a creative bent -- as a child she writes, acts, paints and draws, and plays endless games of Let's Pretend. She becomes a devotee of classical music, and is disappointed that she has no musical ability. She reads an enormous amount even before Thomas Lynn begins sending her books. She also has an athletic bent -- she participates in sports, especially running and football (soccer for Americans) and shows a talent for dance and gymnastics. She eventually attends St. Margaret's College, Oxford, where she seems to be studying literature. She always wears an opal pendant which belonged to her great-grandmother. She loves coffee and ice cream, especially in combination.
Polly has good manners and an acute sense of propriety, although her hero adventures inspire fights at school and even housebreaking. She is soft-hearted (which she finds aggravating), but she can also be as majestic as her grandmother. She is proud in times of trouble, refusing to wear her heart on her sleeve.
Her hero name is Hero, and she is assistant hero-in-training to Tan Coul. Her heroic ability is knowledge.
Hero is usually disguised as a boy. When she first joins Tan Coul as his assistant, he is living a quotidian identity as Thomas Piper, ironmonger of Stow-on-the-Water; it is Hero who reveals to him his true identity. He hires Hero to work in his shop, where she is forced to stay on Leslie's good side, lest he guess that she is a girl or report her to Edna.