Katy is the twelve-year-old eldest child of the Carr family, known for her tall stature and endless physical energy. She possesses a wild imagination and dreams of doing grand deeds when she grows up, such as leading a crusade or becoming a famous artist. In her daily life, however, she struggles with carelessness and frequently tears her dresses or forgets her chores. Though she makes constant resolutions to improve her behavior, her impulsive nature often leads her into minor disasters and schoolyard scrapes.

Helen is a relative of the Carr family who lives with a permanent physical injury that keeps her largely confined to a sofa. Before the children meet her, they expect a mournful and excessively pious figure. Instead, she proves to be cheerful and stylishly dressed, showing deep attention to the happiness of those around her. She maintains a positive outlook and brings personal touches, like her pet vase, to make any room she occupies feel welcoming.

Following the death of Dr. Carr's wife, his sister Aunt Izzie stepped in to run the home. She is a small, precise woman who values neatness and strict obedience above everything else. The messy habits of her nieces and nephews constantly baffle her, leading to frequent scoldings and banned activities. Despite her rigid exterior, she works continuously to keep the large family fed and organized.

Dr. Carr is the benevolent father of the six Carr children and a respected physician in Burnet. He employs a quiet and understanding approach to raising his family, preferring gentle moral instruction over harsh punishment. He actively encourages his children to play rough games and explore the outdoors, an attitude that frequently puts him at odds with his sister's strict domestic standards.

Clover is the second-eldest Carr sibling. She possesses a sunny, sweet-tempered disposition and serves as Katy's most loyal supporter. While she shares her older sister's love for imaginative games, Clover is generally more practical and agreeable, acting as a stabilizing force among the children during their daily activities.

At eight years old, Elsie frequently finds herself excluded from the older girls' secrets and games. This isolation makes her sensitive, and she sometimes responds with a sharp temper when she feels ignored. She desperately wants to be included in Katy and Clover's inner circle but struggles to secure their attention.

Dorry is a solemn six-year-old boy whose thoughts revolve primarily around meals and treats. He documents his daily life in a journal that consists almost entirely of misspelled notes about food. His ultimate fantasy involves having unlimited access to turkey, making him the most food-motivated member of the family.

Joanna, known to everyone as Johnnie, is a sturdy five-year-old who prefers rough-and-tumble play. She embodies the hardy physical spirit her father encourages, caring very little for quiet indoor activities. She acts as Dorry's constant companion and enthusiastically joins in the chaotic games invented by her older siblings.

Philly is the four-year-old baby of the Carr family. Because he is so young, he often misinterprets situations and requires guidance from the rest of the household. He is affectionate and easily influenced by his older siblings' games, sometimes ending up frightened by their dramatic storytelling.

Cecy lives next door to the Carrs and spends enough time at their house to be considered an honorary member of the family. She is a neat, proper girl who keeps her hair smooth and her clothes clean. Her conventional desires for nice dresses and good behavior contrast sharply with Katy's messy, adventurous nature.

Imogen is a new student at Mrs. Knight's school who quickly catches Katy's attention. She affects dramatic manners and constantly tells fabricated stories about romantic encounters with brigands. Her theatrical personality impresses Katy but draws immediate skepticism from the adults in the family.

Mrs. Knight runs the local school attended by the Carr children, maintaining a bitter rivalry with the neighboring Miss Miller's school. She attempts to keep strict order in her classroom and hands down severe punishments when the students invent wild, disruptive games behind her back.

Miss Petingill is a kindly seamstress who works for the Carr family once a week. She watches the children dash through the rain to their secret loft, accepting their chaotic play with gentle amusement rather than the frustration typical of Aunt Izzie.

Alexander is the Carr family's hired man who performs physical labor around the property. He handles various maintenance tasks, from installing play equipment for the children to fetching ladders when doors become accidentally locked from the inside.