Isa is a twelve-year-old musician and twin sister to Jessie. She spends much of her time practicing the violin in the basement, which she has converted into a soundproof studio. As the oldest sibling alongside her twin, she feels a heavy sense of responsibility. She actively worries that her daily violin practice angered their reclusive landlord and caused him to force them out.

Jessie is the twelve-year-old twin sister of Isa and the most science-minded of the siblings. She is bold and rational. She has zero interest in fashion or school dances, preferring to work on building a water wall or fruit batteries. She is deeply attached to her twin but finds herself confused as their adolescent interests begin to diverge.

Hyacinth is the six-year-old, introverted middle sibling of the Vanderbeeker family. She loves crafting, sorting her button collection, and quietly spending time with her pets. Though she struggles with anxiety and fears the grumpy landlord, her family lovingly calls her 'Hyacinth the Brave' to encourage her inner strength.

Oliver is the nine-year-old, only boy in the Vanderbeeker family. He is bookish and imaginative, harboring a deep obsession for high-seas adventures and piracy. He enjoys the privilege of having his own small bedroom built into a closet and is fiercely protective of his family, frequently proposing dramatic or vengeful solutions to their problems.

Laney is the four-year-old baby of the Vanderbeeker family. She brings a chaotic, loving energy to the household, freely distributing hugs and mispronouncing words. She is highly affectionate toward the family's pets and frequently visits the elderly neighbors upstairs to practice tricks and dance.

Papa is the upbeat, hardworking patriarch of the family who has lived in Harlem his entire life. He works part-time as a computer repairman and part-time as the building's superintendent. He proudly wears heavy coveralls while performing dirty maintenance tasks around the brownstone and strives to remain an optimistic support system for his children.

Mama is the practical and industrious matriarch of the Vanderbeekers. She runs a baking business from their busy kitchen while managing the daily schedules of five children. While Papa stays optimistic, she takes a realistic view of their housing situation, feeling the immense pressure to pack their belongings and prepare for the forced move.

Mr. Beiderman is the reclusive, cantankerous landlord of the 141st Street brownstone. He intensely dislikes loud noises and the presence of children, prompting his sudden decision not to renew the Vanderbeekers' lease right before Christmas. Having not been seen by the children in the six years they have lived there, he is treated almost like a mythical monster by the kids.

Miss Josie is an elderly neighbor who lives on the floor above the Vanderbeekers. She is warm and welcoming, frequently inviting the children into her apartment for tea and strawberry jam cookies. She teaches Laney the lindy hop dance and cherishes the bustling presence of the large family in the building.

Mr. Jeet is an elderly neighbor living upstairs with Miss Josie. Having suffered a stroke two years ago, he speaks slowly and moves with difficulty, but he maintains a gentle, loving demeanor. He is particularly close with Laney, who finds comfort in his slow, deliberate way of speaking.

Benny is the son of the neighborhood bakery owners and an eighth-grade classmate of Isa and Jessie. He works at the bakery and has developed a crush on Isa. His awkward attempts to ask her to the upcoming school dance create unexpected tension between the twin sisters.

Allegra is a school friend of Isa's who is highly focused on fashion and the upcoming eighth-grade dance. Her interest in poufy dresses and boys contrasts sharply with Jessie's scientific focus, emphasizing the growing differences between the twins.

Angie is the daughter of a neighboring building's superintendent and one of Oliver's basketball friends. She is a loyal friend who exchanges secret handshakes with Oliver and enthusiastically helps him gather petition signatures to save his home.

Mr. Smiley is the superintendent of a neighboring building on 141st Street. As a fellow building manager, he understands the hard work Papa does and happily accepts the Vanderbeekers' Christmas cookie deliveries.

Mr. Jones is the neighborhood mailman who has known Papa since Papa was a baby. He wears custom-made buttons crafted by Hyacinth on his official uniform and knows the history of the neighborhood's residents.

Uncle Arthur is the children's resourceful uncle who enjoys building things for them. He fosters Oliver's love of adventure books and frequently joins the family for loud holiday celebrations.

Auntie Harrigan is a member of the Vanderbeekers' extended family. She attends their Christmas Eve dinner, bringing a warm presence and offering supportive encouragement to the children during their difficult time.

Mr. Van Hooten is Isa's devoted violin instructor. He holds high standards for her playing and loans her a special heirloom violin to practice on, pushing her to reach her full musical potential.

Mr. Castleman is the co-owner of Castleman's Bakery. He runs the local pastry shop alongside his wife and teenage son, serving the neighborhood their famous cheese croissants.

Mrs. Castleman is the co-owner of Castleman's Bakery. She acts strangely silent and uncomfortable when the children mention they are buying croissants for their landlord, hinting that she knows more about him than they do.

Jimmy L is Oliver's best friend and basketball teammate. They communicate using pirate code names, and Jimmy is deeply distressed to hear the news that Oliver might be moving away from the neighborhood.

Franz is the Vanderbeekers' loud and energetic basset hound. His frequent barking is one of the many things the children fear has upset their reclusive landlord.

George Washington is the Vanderbeekers' pet cat. He freely roams the chaotic brownstone and plays with the other animals.

Paganini is the family's pet rabbit. He is particularly loved by four-year-old Laney, who feeds him carrots and takes him upstairs to practice impressive tricks.