Harriet M. Welsh
An eleven-year-old girl who is an aspiring writer and avid spy. She follows a "spy route" every day after school and takes detailed notes in a notebook. Harriet is a precocious child and a keen observer. She loves routine and follows her own set of habits and patterns of behavior. When her nanny leaves and her notebook is discovered, Harriet is forced to deal with major challenges to her formerly organized life.
Ole Golly
Harriet's eccentric, whimsical nanny. She encourages Harriet's desires to become a writer and is in many ways much closer to Harriet than is Harriet's own mother. While she appears rough and strict, she genuinely loves and cares for Harriet and is invaluable in Harriet's life. She leaves her job after she becomes engaged, as she deems Harriet old enough to live without her care; however, she continues to be an invaluable presence in Harriet's life and comes to her aid when she is in her most vulnerable state.
Sport
One of Harriet's best friends, a boy that loves to play soccer. He lives with his father, a writer who is largely unsuccessful for the duration of the novel. As a result, Sport takes on many of the domestic responsibilities around the house such as the cooking and the cleaning. Sport and Harriet's friendship becomes strained once he sees that Harriet has compared him to an "old woman."
Janie
One of Harriet's best friends, a girl who is passionate about science, much to the disappointment of her mother, who wishes her to behave more like a proper young lady. Janie's mother is a source of comedy for Harriet due to her boisterous personality and her inability to understand her daughter's passions. Janie and Harriet's friendship also become strained once she sees Harriet's scathing review of her love of science and her obsession with the end of the world.
Mrs. Welsch
Harriet's mother, a New York City socialite who loves her daughter but does not spend enough time with her to truly understand her. Her favorite subject in school when she was young was math.
Mr. Welsch
Harriet's father, a wealthy New York City businessman. He is more easygoing than his wife, and has a slightly stronger relationship with Harriet, though he is still rarely home.
Mrs. Golly
Ole Golly's mother, whom Ole Golly takes Harriet and Sport to see at the beginning of the book. She is a large, blundering woman, who is not educated and who is content to spend her entire life living in a tiny house near the beach.
Marion Hawthorne
Harriet's primary nemesis at school, a popular girl who is extremely manipulative and convinces people to do what she wants them to. She is elected class officer, and nominates herself as president of the Spy Catcher Club with the intention of only allowing the people she likes to be members.
Beth Ellen Hansen
The prettiest girl in school, who is also rather timid and is pushed around by Marion. In the end, she is finally able to stand up to her and leave the Spy Catcher Club.
Rachel Hennessey
Another of Marion's cronies. Rachel's garden is used as a location for the Spy Catcher clubhouse.
Carrie Andrews
Another classmate, who Harriet believes has gotten fatter since the last school year. Her father is Harriet's doctor.
Laura Peters
Another of Harriet's classmates, who takes great pride in her beautiful hair. As revenge, Harriet cuts a large chunk of it off.
Pinky Whitehead
One of the three boys in Harriet's class, who Harriet believes is extraordinarily dumb. His mother is dean of the school.
The Boy With the Purple Socks
A new boy in school, whose name is actually Peter. His mother makes him wear purple socks so she can always find him in a crowd, because once when he was younger she nearly lost him at the circus.
The Cook
The Welsch's family cook, who is particularly exasperated by Harriet's antics. Harriet runs smack into her almost every day when she comes home from school.
Mrs. Plumber
A rich old lady who Harriet spies on during her spy route. At the beginning of the book, Mrs. Plumber was determined to remain in her bed all day and not leave it for anybody or anything. When the doctor confines her to bed for the rest of her life, however, she is suddenly averse to the idea, and becomes an active, engaged woman.
Harrison Withers
A man whom Harriet spies on during her spy route. He has 26 cats before the Health Department comes to take them all away, but after this, he manages to acquire a little kitten. He also crafts birdcages for a living.
The Robinsons
A couple whom Harriet spies on during her spy route. They sit in silence and say not a word to each other unless company is visiting, in which case they show them around the house and pretend they have a "perfect" life.
Mama Dei Santi
The matriarch of the Dei Santi family, one of Harriet's spying subjects.
Papa Dei Santi
The father of the Dei Santi family that Harriet spies on, and owner of the Dei Santi grocery store.
Bruno Dei Santi
The oldest of the Dei Santi children.
Fabio Dei Santi
One of the Dei Santi children, who is eighteen years old and trying to be independent. He decides he does not want to work the grocery business and gets a job as a salesman.
Maria Elena Dei Santi
One of the Dei Santi children.
Miss Elson
Harriet's homeroom teacher.
Mrs. Gibbs
Janie's mother, who wants Janie to be more ladylike. She suggests that Janie and Harriet attend dancing school.
Mr. Rocque
Sport's father, who is an aspiring writer and unfortunately has a lot of trouble publishing his work and making money for him and Sport to live on. Sport does most of the household work for them, because his father will not do it.
Miss Angela Whitehead
The dean of the Gregory School, who is Pinky Whitehead's mother.
Little Joe Curry
The Dei Santi's delivery boy, who is friends with Mr. Waldenstein. He always eats a ton at the grocery store, and for a time, the Dei Santis fire him because of it.
Mr. Waldenstein
Ole Golly's fiancé, a delivery man who was recently promoted to cashier. He was once a wealthy jewelry business owner with a wife and son, but after realizing he was unhappy, he renounced all his material wealth and started over.
Miss Berry
The dance teacher, who assigns roles for the Christmas pageant.
Miss Doge
The teacher who is in charge of measuring students for costumes for the Christmas pageant.
Dr. Andrews
Carrie Andrews' father, who is the Welschs' doctor. He tells Mrs. Welsch that there is nothing wrong with Harriet, but confesses that Carrie told him something about a notebook, and that might be the problem.
Dr. Wagner
The psychiatrist who Harriet visits with her mother. He also says there is nothing wrong with Harriet, but recommends that she be given a project to keep her mind occupied.
Mrs. Hennessey
Rachel's mother. Harriet leaves her a note that says the children are not at her house because they like Rachel—instead, they are only there to eat her mother's homemade cake.
