Christopher Boone - The 15-year-old narrator and protagonist of the novel. He has an unspecified condition that is implied to have autism or Asperger's syndrome. Christopher has exceptional skills in math and science but struggles with social interactions and interpreting emotions. He embarks on an investigation to find out who killed the neighbor's dog Wellington.

Ed Boone - Christopher's father, who initially lies to Christopher about his mother's whereabouts. He killed his neighbor's dog Wellington after an argument with the neighbor Mrs. Shears, with whom he was having an affair.

Judy Boone - Christopher's mother, who had an affair with Mr. Shears and left the family to live with him in London before returning.

Mrs. Shears - The neighbor whose dog, Wellington, was killed. She had an affair with Ed Boone, Christopher's father.

Mr. Shears - Mrs. Shears' husband, with whom Judy Boone (Christopher's mother) had an affair and lived in London for some time.

Siobhan - Christopher's teacher who gives him the assignment to write the book that forms the novel's narrative.