Kristy is a twelve-year-old, outgoing seventh-grader who lives in Stoneybrook with her mother and three brothers. She gets the idea to start the Baby-Sitters Club after seeing her mother frantically search for last-minute childcare. As the club's founder and president, she demonstrates strong leadership and business skills. She struggles to accept the presence of her mother's new boyfriend, preferring her family dynamic to remain unchanged.

Claudia is an artistic, fashion-forward seventh-grader who lives across the street from Kristy and Mary Anne. She serves as the vice president of the Baby-Sitters Club, hosting meetings in her bedroom because she has her own private telephone line. She frequently clashes with her parents over her schoolwork and feels overshadowed by her highly academic older sister.

Mary Anne is a quiet, reserved girl who lives next door to Kristy. Her mother died when she was an infant, and she lives under the strict rules of her overprotective father. She serves as the secretary of the Baby-Sitters Club, using her excellent listening skills to manage their scheduling and record-keeping.

Stacey is a stylish, cosmopolitan girl who recently relocated to Stoneybrook from New York City. She takes on the role of treasurer for the Baby-Sitters Club, utilizing her strong budgeting skills. She deflects questions about her past and avoids eating certain foods, leading some members of the club to suspect she is hiding something.

Kristy's Mom is a divorced, working mother who balances her career with raising four children. She acts with patience and understanding toward Kristy's impulsive behavior. She dates a wealthy man named Watson, bringing new changes into her family's routine.

Watson is a wealthy, divorced man dating Kristy's mother. He lives in a large home and shares custody of his two young children, Karen and Andrew. He makes consistent, friendly efforts to connect with Kristy and her brothers, despite Kristy's initial hostility.

David Michael is Kristy's six-year-old brother. His frequent need for childcare serves as the catalyst for the creation of the Baby-Sitters Club. He is agreeable and highly accustomed to being watched by various teenage girls.

Charlie is Kristy's eldest brother. He plays football and stays busy with his own teenage life, generally maintaining an easygoing attitude toward the changes happening in his family.

Sam is Kristy's older brother. He appears mostly in the background of the Thomas household until he meets the new girl from New York, Stacey, and decides to hang around the house more often.

Karen is Watson's highly talkative five-year-old daughter. She possesses an incredibly active imagination and has strong opinions about her parents' divorce and her father's potential remarriage.

Andrew is Watson's young son. He stands in stark contrast to his loud sister, preferring to remain quiet and expressing anxiety over the shifting dynamics of his parents' divorce.

Janine is Claudia's older sister. She is considered a certified genius by her family, which places a heavy burden of expectation on Claudia. She focuses heavily on academics and intellectual pursuits.

Mr. Spier is Mary Anne's father. Having lost his wife when Mary Anne was a baby, he raises his daughter alone using a very strict set of rules to keep her safe and modest.

Mrs. Porter is Watson's next-door neighbor. She maintains her garden and dislikes it when animals trample her flowers. Karen firmly believes she is a witch named Morbidda Destiny.

Boo-Boo is Watson's overweight, grumpy cat. He acts aggressively toward anyone he does not know and frequently wanders into the neighbor's yard, causing trouble.

Louie is the Thomas family dog. He spends time with the children when they play outside near their home.