Lolly is a 12-year-old boy living in the St. Nicholas Houses in Harlem. He struggles with heavy grief following the recent shooting death of his older brother, Jermaine. To cope with his complicated feelings and the increasing neighborhood pressure to join a street crew, he channels his energy into building elaborate Lego models and expanding an original plastic city he calls Harmonee.

Casimiro, often called Vega, is Lolly's 12-year-old best friend, a Dominican boy with a playful, joking demeanor and thick, bushy hair. He plays the violin and provides a cheerful contrast to Lolly's quiet grief. Like his best friend, he faces increasing harassment and pressure from older boys in the neighborhood to abandon his childhood hobbies and join a local street crew.

Rose is a tall, quiet girl in Lolly's after-school program who rarely speaks and strictly adheres to her own routines. She possesses a remarkable memory and spatial awareness, allowing her to build incredibly detailed Lego replicas of the St. Nicholas Houses purely from observation. She shares a profound connection of loss with Lolly, channeling her internal world into physical structures.

Jermaine is Lolly's older brother who was shot and killed outside a Bronx nightclub prior to the events of the story. Before his death, he worked at a local barbershop and increasingly associated with a neighborhood street crew. His memory serves as a constant presence in the narrative, representing both a fiercely protective older sibling and a cautionary example of street violence.

Mr. Ali is a licensed social worker and the director of the after-school program at the community center. He was born with Apert syndrome, which affects his facial structure, but he commands immense respect from his students. He uses a direct, patient approach to help the children process their emotions and stay out of trouble.

Sue-ellen is Lolly and Jermaine's mother. She works as a security guard downtown, collects Pez dispensers, and is physically imposing but generally gentle. She carries deep emotional wounds from losing Jermaine, shifting between strict, fearful parenting to keep Lolly safe and profound sadness over her fractured family.

Yvonne is Sue-ellen's long-term girlfriend who works as a custodian at Tuttle's Toy Emporium in Rockefeller Center. She acts as a supportive aunt figure to Lolly, frequently spoiling him with meals out and attempting to lift his spirits following his brother's death by bringing home materials for his hobby.

Daddy Rachpaul is Lolly and Jermaine's father, a muscular construction worker who also performs as a clown at children's parties on weekends. He holds traditional, rigid views on masculinity and physical toughness, frequently pushing his sons to physically "man up." He lives separately from the family and struggles to connect emotionally with Lolly.

Steve is a 20-year-old neighbor and aspiring filmmaker who managed to avoid the neighborhood street crews while growing up in the projects. As Jermaine's childhood best friend, he serves as an alternative role model for Lolly, actively encouraging the young boy to pursue his creative and architectural talents rather than street life.

Sunnshyne, who goes by Sunny, is a girl in Lolly's after-school program. She frequently acts out by bullying other students, particularly Rose and Casimiro Vega, though she harbors a secret crush on Lolly. She fancies herself an amateur detective, running an investigative agency with her best friend.

Frito is Casimiro Vega's older cousin who is actively involved in a local street crew. He frequently pressures Casimiro to abandon his innocence and join his group, arguing that crew membership is necessary for protection against neighborhood bullies.

Harp is an older boy in the neighborhood who, alongside his friend Gully, routinely harasses younger children. He targets Lolly and Casimiro Vega, stealing their possessions and trying to force them into a rival street crew through physical intimidation.

Gully is Harp's constant companion and fellow neighborhood bully. He assists in threatening younger kids in the St. Nicholas Houses area, acting as the physical muscle in their efforts to steal phones and enforce gang territory lines.

Rockit is an older neighborhood resident who previously worked at the barbershop with Jermaine. He is deeply involved in the local street crew lifestyle and attempts to keep an eye on Lolly after Jermaine's death, offering a misguided form of protection out of loyalty to his deceased friend.

Dr. Betty Green is Rose's highly educated grandmother and primary guardian. She fills their home with classic literature and poetry, providing Rose with stability and an unconventional education outside of the standard public school expectations.

Harold Tuttle is the owner of Tuttle's Toy Emporium in Rockefeller Center. He runs a massive, three-story business and manages a large staff, maintaining strict professional boundaries regarding his inventory and the operations of his stores.

Mr. Jonathan is a close gay friend of Sue-ellen who acts as a supportive, uncle-like figure to Lolly. He faces prejudice from Daddy Rachpaul, who unfairly blames him for the end of his relationship with Sue-ellen.

Nicky is a wild, grey-and-white coyote that has wandered down into Harlem looking for food and shelter. The animal becomes a symbol of survival and displacement for the neighborhood children, who occasionally leave food out for it rather than turning it in.

April E. is Sunnshyne's best friend and constant companion. She follows Sunny's lead in their amateur detective schemes and helps her attempt to tame the wild wildlife around their neighborhood.

Heike is Daddy Rachpaul's current girlfriend. She is an easily startled woman who feels entirely out of place when visiting Lolly's home, making her an easy target for Lolly's displaced anger during a tense family visit.