Nine-year-old Oliver is an energetic boy with a competitive streak. He spends the early days of summer harboring jealousy over his older sister's new phone and his classmate Herman's expensive mountain bike. When a harsh argument with his sister severely distresses his elderly neighbor Mr. Jeet right before a medical emergency, Oliver feels a crushing sense of guilt. He channels his anxiety into a secret mission, rallying his siblings to transform a trash-filled lot into a community garden as a recovery gift.

Almost 13 years old, Jessie is the logical, scientifically-minded sister of the Vanderbeeker family. With her twin sister away at orchestra camp, Jessie steps into a more prominent caretaking role for her younger siblings. Initially skeptical of the garden project because of the practical obstacles, she soon applies her sharp organizational skills to the endeavor, drafting liability waivers and researching plant care.

Seven-year-old Hyacinth is the emotional compass of the Vanderbeeker family. She possesses a deep, intuitive empathy for people, animals, and the natural world. Although she initially fears the abandoned lot is haunted, she feels a unique pull toward a large silver maple tree inside, which she names the "Silver Queen." Her compassionate nature allows her to see past people's rough exteriors.

At five and a quarter years old, Laney is the youngest Vanderbeeker sibling. She interprets the world literally and acts with pure, straightforward affection. When the garden project begins, she immediately pledges her entire life savings of $43. Her innocent curiosity leads to major discoveries in the dirt, and her reckless acts of love often cut through the adults' strict rules.

Mr. Beiderman is the Vanderbeekers' third-floor landlord. He is a grieving recluse who has not left the building in six years following the tragic deaths of his wife and teenage daughter, Luciana. He works for the Landmarks Preservation Commission from home. When he learns the children are working on the lot next to the church—his late daughter's former preschool—he begins to quietly observe their progress from his window.

Mr. Huxley is Herman's father and the church treasurer. Pragmatic and entirely finance-focused, he immediately rejects the children's request to build a garden, citing liability concerns. Driven by the church's need for repair funds, he views the overgrown land purely as a disposable asset and works to sell it to condominium developers.

Isa is Jessie's 12-year-old twin sister. Although physically absent for most of the story while attending her music program, she remains a constant presence in her siblings' lives through phone calls and text messages. She acts as a sounding board for the family, offering level-headed advice and encouragement when the garden project faces obstacles.

Herman is Oliver's classmate and the son of the church treasurer. Initially presented as a boastful boy who flaunts his unearned wealth, he frequently dismisses the abandoned lot as a "toxic waste dump." However, his abrasive exterior hides a lonely child whose parents are emotionally distant. A surprising encounter reveals he has a creative, gentle side.

Miss Josie is the Vanderbeekers' warm, nurturing second-floor neighbor. As a former educator at the New York Botanical Garden, she maintains a deep love for plant life and first plants the idea of transforming the local derelict lot into a community space. She faces immense emotional strain when her husband suffers a severe medical emergency.

Mr. Jeet is a dapper, gentle man who lives on the second floor with his wife. Already recovering from a previous stroke, he suffers a second, more severe stroke early in the summer after being distressed by an argument between Oliver and Jessie. His hospitalization and need for a "healing garden" become the primary motivation for the children's secret project.

Mama Vanderbeeker is the loving, hardworking mother of the five Vanderbeeker children. She runs a baking business while managing the chaotic household. Deeply affected by Mr. Jeet's medical emergency, she processes her anxiety by enforcing a strict health-food regimen at home, replacing treats with salads and confiscating unhealthy pantry items.

Papa Vanderbeeker is the reliable, supportive father of the family. He works long hours to provide for his large household and struggles to manage the financial strain of the children's summer camps. Though largely unaware of the secret garden project for much of the summer, he provides a stable foundation for his kids during a frightening time.

Angie is Oliver's loyal friend and the daughter of Mr. Smiley, a local building superintendent. Resourceful and quick-thinking, she suggests hosting a sidewalk sale to raise funds for the garden. She frequently assists the Vanderbeekers in their grueling physical labor and provides crucial, albeit unsanctioned, materials for the lot.

Orlando is Mr. Jeet's grandnephew who has recently moved to Harlem permanently from Georgia. He is a skilled gardener who misses the wide, starry skies of his former home. He quickly integrates into the Vanderbeekers' project, offering invaluable horticultural knowledge and helping to save Miss Josie's dying seedlings.

Benjamin is the son of the local bakery owner. He is friendly with the Vanderbeeker siblings and frequently supports their efforts by supplying them with boxes of pastries to keep their energy up while they work in the dirt.

Triple J is the pastor of the local church that owns the abandoned lot. Though he appears generally supportive of the neighborhood, he is highly distracted by a sudden family emergency that requires him to leave town for a month, leaving the property's fate in the hands of the treasurer.

Mr. Smiley is Angie's father and the superintendent of a large apartment building. When he discovers the children have repurposed abandoned soil from his courtyard without permission, he issues a strict, labor-intensive punishment.

Luciana is Mr. Beiderman's deceased teenage daughter, who was killed alongside her mother in a tragic accident six years prior. She once attended preschool at the church next door, and her memory heavily influences her father's isolation. A small wooden box bearing her name is unearthed in the lot.

Paganini is Laney Vanderbeeker's beloved pet rabbit. The animal inadvertently causes chaos in the hospital ward but ultimately brings immense joy to a recovering patient.

Hiba owns the neighborhood hardware store. When she learns of the children's ambitious plan to clear the lot, she generously donates a linden tree to their cause, establishing the first act of community support for the garden.