The oldest of the Smithfork children, CJ takes on the responsibility of watching his younger siblings when their parents are occupied. He struggles to adapt to the family's sudden wealth and their relocation from Brooklyn, feeling disconnected at his new school. Protective and sometimes overbearing, he takes the lead in exploring the strange architectural mysteries of their new home.

The middle Smithfork child, Brid is resourceful and highly involved in deciphering clues scattered around the city. She acts as a supportive confidante to her older brother, CJ, and frequently invents cover stories to keep their investigations secret from their nanny. While she sometimes antagonizes Patrick, she possesses a natural ability to mediate conflicts and understand others' perspectives.

The youngest of the sleuthing Smithfork siblings. Though he has dyslexia, which makes reading written clues difficult, Patrick has an exceptional eye for detail and notices physical anomalies that his older siblings miss. He is brave and willing to crawl into tight spaces like hidden dumbwaiters to uncover secrets.

The Smithfork family's downstairs neighbor and the daughter of the wealthy Mr. Post, who originally owned the entire undivided apartment. She is initially skeptical of the children's active imaginations and attempts to distance herself from her family's painful history. Her father's poetry book gradually reignites her interest in the legacy he left behind.

The Smithfork children's father, who transitions from a basement game developer to a wealthy executive after his game, PeeWee, becomes a massive hit. His sudden success leaves him with less time to spend with his family, causing friction with his children who miss their old, simpler life in Brooklyn.

The mother of the Smithfork children. She finds herself increasingly busy with restoring the building and managing their new lifestyle. Though often distracted, she cares deeply for her children's well-being and attempts to rebuild their close-knit family dynamic when she realizes they are drifting apart.

Eloise Post's brother, who disappeared mysteriously when he was a child. Following the high-profile kidnapping of Charles Lindbergh's son, the Post family lived under strict guard, but Julian still vanished after taking a trip with his father. His disappearance fractured the Post family and left Eloise with lingering questions about his fate.

The wealthy former owner of the Smithforks' apartment building. A lover of puzzles and codes, he set up a complex treasure hunt around New York City using a book of poetry to lead his children to their inheritance. His actions in the past cast a long shadow over the current residents of his former home.

A neighbor to the Smithforks who lives in one of the building's other apartments. She attends a formal English boarding school and represents the elite social class the Smithforks have suddenly joined. Polite and helpful, she provides the new residents with background information on the building's history.

The Smithforks' neighbor and Lily's brother. Accustomed to a formal, wealthy lifestyle heavily contrasting the Smithforks' Brooklyn roots, he helps the new residents by providing access to the basement servants' quarters and sharing a photograph of the original Post family.

The Smithfork family's nanny, hired to help look after the children now that both parents are frequently busy. She is primarily responsible for the baby, Carron, and tries her best to keep track of the older children, though they frequently deceive her to sneak out and investigate clues.

The youngest member of the Smithfork family. As a baby, she is mostly cared for by the nanny, Maricel, or her mother, remaining oblivious to the complex treasure hunt her older siblings are undertaking.

A worker in the Smithforks' apartment building who operates the elevator and also serves as Eloise's personal driver. He is knowledgeable about the building's residents and history, though he can be tight-lipped when the children are sneaking around the basement.

CJ's new school friend who accidentally helps further the investigation by recognizing dot writing codes above the letters in the poetry book. He acts as a moral compass for CJ, challenging him when he attempts to lie about skipping school.

The head librarian at the local research library. After calculating the late fees from 1937, she provides the children with a special package left by Mr. Post, initiating their city-wide scavenger hunt.