Christopher is a 15-year-old boy who has autism and a profound talent for mathematics and science. He relies on strict routines and logical rules to move through a world that often feels overwhelming, especially when trying to understand human emotions and metaphors. Driven by a desire for truth and order, he decides to investigate the murder of his neighbor's dog despite his father's explicit instructions to stay out of other people's business.

Ed owns a small heating repair and maintenance business and serves as the sole caregiver for Christopher. He strives to protect his son and accommodate his strict routines, though he sometimes loses his temper under the pressure of single parenthood. He strongly opposes Christopher's decision to investigate the dog's death and demands that he stop immediately.

Judy is Christopher's mother. Two years prior to the events of the novel, she was hospitalized for a sudden heart problem and passed away. According to Christopher's memories, she was an active, health-conscious woman who loved her son deeply but sometimes struggled with the intense patience required to raise a child with unique behavioral needs.

Siobhan is a tutor at Christopher's school who specializes in helping students manage social and academic challenges. She understands Christopher's mind better than most adults and provides him with clear, literal instructions rather than confusing figures of speech. She actively encourages him to write a book about his detecting experiences.

Eileen Shears is a neighbor living across the street from the Boones and the owner of the murdered poodle, Wellington. Following the death of Christopher's mother, she spent significant time at the Boone household, helping Ed by cooking and tidying up. However, she has since become distant and reacts with hostility to Christopher's sudden investigation.

Roger Shears is Eileen's estranged husband who left the neighborhood two years prior to the events of the novel. Christopher identifies him as the prime suspect in Wellington's murder, reasoning that a divorced person might harbor enough residual hatred to return and kill their former spouse's dog.

Mrs. Alexander is an elderly neighbor who lives on the other side of Mrs. Shears's house. She owns a dachshund named Ivor and tries to be friendly to Christopher, offering him tea and biscuits. She possesses knowledge about the neighborhood's past and gently attempts to steer Christopher away from certain painful truths while still wanting to help him.

Rhodri works as an assistant in Ed Boone's heating and maintenance business. He is a boisterous man who wears dirty work clothes that carry a distinct odor. He frequently prompts Christopher to solve difficult math problems in his head for his own amusement.

Toby is Christopher's pet rat and constant companion. Christopher appreciates Toby because he is clean and easy to understand, offering an uncomplicated presence that does not demand confusing social interaction. Toby represents one of the few living creatures Christopher feels entirely comfortable caring for.

Sandy is a sandy-colored golden retriever puppy. For Christopher, who finds dogs much easier to understand than human beings, a canine companion offers a straightforward and reliable connection based on basic needs and clear behaviors.

Wellington is a French poodle belonging to Eileen Shears. His sudden and violent death serves as the catalyst for the entire story, prompting Christopher to step outside his comfort zone and investigate the neighborhood to find the culprit.

The local police officers frequently intersect with Christopher's life. While Christopher respects their uniforms and the order they represent, the officers are generally untrained in how to communicate with someone who has autism. This disconnect often leads to chaotic misunderstandings and physical altercations when they attempt to touch or grab him.

Mr. Jeavons is the psychologist at Christopher's special-needs school. He attempts to analyze Christopher's rigid habits, such as his system of deciding if a day is a "Good Day" or a "Black Day" based on the colors of passing cars. Christopher often finds Mr. Jeavons's assessments of his intelligence to be slightly inaccurate.

Mrs. Gascoyne is the headmistress at Christopher's special-needs school. She initially refuses to let Christopher take his A-level math exams due to financial and administrative concerns, fearing it sets a difficult precedent, but she ultimately yields after Ed Boone fiercely argues on his son's behalf.