Jacob Hunt is eighteen years old, lives with his mother Emma and brother Theo, and has Asperger's syndrome, then considered a form of high-functioning autism. (The 2013 edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published after the novel, replaced Asperger's syndrome with diagnoses of autism spectrum disorder on a severity scale.) Jacob lives by a structured schedule and feels comfortable only when all of his daily activities are planned; Emma indulges him by bending her and Theo's lives around his needs. Jacob is obsessed with forensic analysis to the point of compulsion.  His need to engage in structured, single-minded activities, as well as his obsession with detail, often frustrates his mother and infuriates his brother. The novel opens with Jacob setting up a crime scene (in which he is the victim) for his mother to solve. Jacob is later accused of murdering his tutor Jess Ogilvy. Eventually it is revealed that Theo snuck into a house Jess was housesitting at and startled her, causing her to accidentally hit her head on the sink and die. When Jacob showed up for tutoring, he staged a crime scene to make it look like Jess's boyfriend Mark Maguire had committed the murder and then Mark had tried to make it look like a kidnapping. Jacob states that he did this to take care of his brother, in accordance with a "house rule" set down by Emma, and that he would do it again.
Jacob Hunt - Eldest son of Emma and Theo's brother. He has Asperger's syndrome and he is accused of murdering Jess Ogilvy, his social skills teacher.
Emma Hunt - Single mother of Jacob and Theo Hunt. She works as a columnist for their town's local newspaper. She has devoted herself to her eldest son, Jacob, as a consequence of his disability. She creates "House Rules" which stipulate the following: Clean up your messes, tell the truth, brush your teeth twice a day, don't be late for school, and take care of your brother; he's the only one you've got.
Theo Hunt - 15-year-old brother of Jacob, claims himself to be independent and rebellious. 
Rich Matson - The lead detective on Jess's case.