Rosalind is the 12-year-old eldest Penderwick sister who assumes heavy responsibility for her younger siblings. She feels deeply protective of her family's current dynamic and strongly opposes the idea of her widowed father dating again. Her natural leadership skills drive her to organize her sisters into taking action regarding their father's romantic prospects. As she approaches her teenage years, she balances her mature caretaking duties with the confusing emotions of early adolescence.

Skye is the intelligent and pragmatic 11-year-old second daughter. She possesses blond hair and blue eyes, contrasting with her dark-haired sisters. With a strong interest in mathematics and astrophysics, she values logic over emotional displays and constantly works to manage her quick temper, especially on the soccer field. Her desire for academic excellence occasionally leads her to make questionable choices regarding school assignments.

Ten-year-old Jane is the highly imaginative, bookish third Penderwick sister. She aspires to be a famous author and often struggles to separate fiction from reality, preferring to write about her own invented characters rather than actual historical events. She speaks with formal, dramatic flair and remains highly sensitive to the feelings of those around her. She frequently uses the Enchanted Rock in Quigley Woods to dispose of cursed objects or attempt magical summons.

Batty is the four-year-old youngest member of the family. Having lost her mother shortly after her birth, she relies heavily on her father and older sisters for guidance. She explores her neighborhood with her faithful dog, utilizing a keen intuition that often leads her to notice things her older siblings miss. She approaches new adults cautiously but warms quickly to those who show genuine kindness.

Martin is a mild-mannered, introverted botany professor at Cameron University. As a devoted single father, he creates a warm, loving home for his daughters while struggling with lingering grief over his wife's passing. He frequently quotes Latin phrases and relies on his eldest daughter to help manage the household.

Iantha is a widowed astrophysics professor who has recently moved to Gardam Street. She possesses a quiet, nurturing demeanor and shares Martin's academic introversion. Her expertise in dark matter and gentle parenting style quickly endear her to the neighborhood children.

Claire is Martin's sister and the girls' aunt. She acts as a catalyst for the family's changing dynamic by delivering a long-held letter from her late sister-in-law. She firmly believes her brother needs adult companionship and actively encourages him to step out of his comfort zone.

Lizzy is Martin's late wife and the mother of the four Penderwick girls. Though she passes away from cancer when Batty is a newborn, her presence heavily influences the family's current lives. She leaves behind a posthumous letter intended to ensure her husband does not spend the rest of his life alone.

Tommy is a seventh-grade boy living on Gardam Street and a lifelong friend of Rosalind. He enjoys running football training exercises and studies Russian. He holds unexpressed feelings for Rosalind, which creates friction when she misinterprets his attempts to spend time together.

Nick is Tommy's older brother and a high school student in Cameron. He occasionally babysits for the neighborhood families and plays football. He often attempts to give his younger brother unsolicited relationship advice.

Ben is Iantha's toddler son. Having lost his father before he was born, he forms quick attachments to his new neighbors and brings a sense of joyful chaos to the adults around him.

Anna is Rosalind's best friend. Because she has experienced several stepmothers, she validates Rosalind's fears about Martin dating and eagerly helps scheme ways to disrupt his romantic endeavors.

Lara is Anna's ice-skating coach. She wears rabbit-fur coats, talks loudly over classical music concerts, and represents everything Martin would find highly disagreeable in a companion.

Norman is a former colleague of Iantha's from the university. He harbors intense professional resentment and carries deep grievances regarding academic research and intellectual property.

Hound is the large, enthusiastic family dog of the Penderwicks. He frequently accompanies Batty on her outdoor expeditions and struggles to contain his excitement when participating in the girls' elaborate games.

Asimov is Iantha's large orange cat. He frequently escapes onto neighboring roofs, prompting rescue missions that inadvertently force the neighborhood residents to interact.

Trilby is an eighth-grade girl at Tommy and Rosalind's school. She actively pursues Tommy's attention, much to Rosalind's private frustration.

Jeffrey is a mathematically and musically gifted boy the Penderwick sisters befriended during a previous summer vacation. He currently attends a boarding school in Boston but maintains a close correspondence with the family.

Melissa is the captain of Cameron Hardware, the rival soccer team to Antonio's Pizza. She plays aggressively on the field and harbors a secret jealousy regarding Skye's popularity.

Pearson is a boy in Skye's sixth-grade class. He develops a crush on Skye after watching the school production, which causes friction with other girls in their grade.