Oskar is a precocious, intelligent nine-year-old living in Manhattan who struggles with fear and depression, which he calls "heavy boots," following his father's death on September 11. He relies on his wide-ranging interests, such as entomology, and his vivid imagination to cope with the unpredictability of the world. Finding a mysterious key in a blue vase in his father's closet gives him a new mission and a reason to explore New York City.

Oskar's Grandmother lives in an apartment across the street from her grandson and serves as his closest confidante, providing comfort and stability when he feels overwhelmed. She grew up in Dresden, Germany, and was the only member of her family to survive the World War II bombings. She writes letters to Oskar detailing her past, her deep love for her sister Anna, and her complex marriage.

Thomas Schell is a survivor of the Dresden bombings during World War II who emigrated to the United States. The profound trauma of losing his first love, Anna, gradually robbed him of his ability to speak, leading him to communicate through notebooks and the words "YES" and "NO" tattooed on his hands. He struggles to move on from his past, keeping his distance from those who care for him.

Oskar's Mother is a widow trying to manage her own immense grief while raising her highly sensitive son. She maintains a supportive but slightly distant presence in Oskar's daily life, allowing him the independence to roam the city while quietly keeping a watchful eye on him. She attends a support group where she meets new acquaintances, which bothers Oskar.

Thomas Schell was a jeweler who died in the September 11 terrorist attacks, leaving his family with an empty coffin and many unanswered questions. Before his death, he fostered Oskar's curiosity by teaching him about science, sharing stories like the fable of the Sixth Borough, and sending him on educational scavenger hunts. He maintained an open, honest relationship with his son, encouraging him to view the world thoughtfully.

Mr. Black is a cheerful elderly man who has lived through the entire 20th century and claims to have fought in multiple wars. He resides in a fascinating, crowded apartment filled with artifacts from his travels and keeps a bed made from a tree. Having not left his apartment or turned on his hearing aids for decades, his isolation ends when Oskar invites him to join the search for the lock.

Anna is the older sister of Oskar's Grandmother who lived in Dresden, Germany, before the World War II bombings. She represents a period of innocence and intense, youthful love for the people who survived her. Her memory looms large over the lives of her sister and Thomas, creating a void that neither fully knows how to fill.

Abby Black is a woman in her mid-forties whom Oskar visits during his alphabetical search of New York City residents. She lives in Greenwich Village and is dealing with a tense personal situation with her husband when Oskar arrives. Although she insists she knows nothing about the key, Oskar intuits that she holds her own quiet struggles.

Ron is a man who meets Oskar's Mother at a support group for individuals dealing with profound grief. He lost his own wife and daughter in a sudden car accident. He acts as a polite presence in the household, but Oskar views him as an unwelcome intrusion.

Dr. Fein is a psychologist tasked with evaluating Oskar's emotional state following his father's death. He asks probing questions about Oskar's feelings and suggests hospitalization when he observes concerning behaviors. He operates from a clinical perspective that frustrates Oskar.

Jimmy Snyder is a classmate of Oskar who plays the lead role in the school's production of *Hamlet*. He frequently acts as a bully, using his confidence to taunt Oskar on the playground and creating an antagonistic environment at school.

Stan is the doorman at the apartment building where Oskar and his family live. He observes the comings and goings of the residents, serving as a familiar, everyday presence in Oskar's immediate neighborhood.

Ruth Black is a resident who lives secretly within the Empire State Building, sleeping in its storage rooms and observation decks. She serves as a knowledgeable, unconventional guide to the skyscraper, possessing an intimate understanding of its history and architecture.

Abe Black is one of the individuals on Oskar's alphabetical list of people with the surname Black. He resides near Coney Island and briefly shares a thrilling, fear-facing experience with the young boy before directing him to his next stop.

Aaron Black is the first person Oskar visits on his alphabetical search through New York City. He lives in Queens and is too sick to come downstairs, forcing Oskar to face his hesitation about going into unfamiliar buildings.