Christopher Boone is the 15- (and 3 months, 2 days) year-old protagonist of the novel. Christopher suffers from an unnamed disability, somewhere on the autism spectrum, most likely Asperger's syndrome. This disorder causes Christopher to have behavioral issues like lashing out violently when someone touches him and wetting himself when he doesn't have access to a clean and familiar toilet. Christopher is highly intelligent academically, particularly in areas like math and science, but he struggles to read the physical and emotional social cues of those around him and panics in unfamiliar situations. He notices each tiny detail in his surroundings, sometimes overwhelming his brain. Christopher is able to tap into his own resilience and bravery when he feels threatened by his father, and he decides to run away to London. Despite his many quirks, Christopher is able to achieve his goals. Through his journey he learns he is capable of more than he ever thought possible if he wants it enough. The truth is the most important thing to Christopher because the truth is simple and direct, just as Christopher wants everything in life to be. However, over the course of the novel, Christopher is confronted by the idea truth is not always absolute and can be very complicated.
Ed Boone is Christopher's father who takes on the challenge of raising Christopher alone when his wife runs off with another man. He owns a modest business, a heating maintenance and boiler repair company, with only one employee. Mr. Boone displays a complicated internal life: although his ex-wife admires his patience and laid-back approach to Christopher's disability, taking the challenges as they come adds to the pressure he feels when dealing with Christopher alone. He lies to Christopher about his mother's death and prevents the two of them from communicating, and he murders the dog Wellington when his affair with Mrs. Shears goes sour. He gets violent with Christopher for disobeying him, yet he also fights for his son's right to take the university entrance math exam (A-levels) and is tremendously proud of Christopher's abilities. He shows signs of genuine remorse for his questionable actions and is devastated at the prospect of losing his son. The dog he buys Christopher as a peace offering symbolizes how much he understands and loves his complicated and frustrating son and foreshadows reconciliation may be possible between them.
Judy Boone is Christopher's mother who had an affair with a neighbor and abandoned her disabled son because she could not cope with his needs. Mrs. Boone tends to be more emotional and more easily frustrated than her ex-husband, experiencing a wider spectrum of feelings from fierce love and pride in Christopher to despair she will never be able to touch him or take care of him. Her abandonment is not a simple, straightforward choice. She wants to remain a part of Christopher's life, as evidenced from her numerous letters to him, yet is incapable of dealing with Christopher's many behavioral problems. Despite her history she does not hesitate to care for Christopher when he arrives in London needing her, and she is willing to walk away from the (albeit unhealthy) relationship with Mr. Shears. Though flawed, she loves her son and wants what is best for him.
Roger Shears is the Boones' neighbor who has an affair with Mrs. Boone and whisks her off to start a new life in London. Readers' first impression of Mr. Shears is as a mocking bully, indicating an unpleasant and mean-spirited disposition, which further comes out in his treatment of Christopher, whom he dislikes and resents. Mr. Shears is the most villainous character in the novel, demonstrating no redeeming qualities.
Mrs. Alexander is a neighbor of the Boones, who accidentally reveals to Christopher his mother had an affair with Mr. Shears. Mrs. Alexander shows a great deal of kindness toward Christopher and is undeterred by his disability. She is compassionate toward his differences rather than judging him for them, even when he fails to acknowledge her attempted hospitality. Though something of a nosey neighbor, Mrs. Alexander tries to do the right thing and sends Christopher back to his father to work out their problems rather than trying to get personally involved.
Eileen Shears is the Boones' neighbor, whose dog, Wellington, is murdered with a garden fork. The nature of her previous relationship with Mr. Boone is unclear. At the least she was sympathetic toward Mr. Boone and helped him with household chores, and it is possible they became romantically entangled in the wake of their spouses' affair. However, despite helping take care of Christopher after his mother leaves, Mrs. Shears is easily able to walk away from Mr. Boone. Readers see very little of her within the forward motion of the novel except for a few encounters when she clearly shows distrust and distaste for Christopher. She also maintains a hardened resentment toward Mrs. Boone for running off with her husband. Yet at the end of the novel she has the chance to press charges against Mr. Boone for Wellington's murder and she doesn't, perhaps demonstrating a streak of compassion or kindness that rounds her out as a complex character.
Siobhan has been one of Christopher's teachers at his special school for eight years, and he describes her as having long blond hair and green plastic glasses. Though Christopher does not explain the extent of their relationship, Siobhan seems to be a one-on-one assistant to Christopher, part teacher and part therapist. She exhibits a great deal of patience with Christopher, trying to help him gain a better grasp of the world around him, worrying about him when things are particularly difficult for him, and encouraging him to write his book as a way of expressing himself.