Matthew is a twelve-year-old boy living in a quiet cul-de-sac outside London. He confines himself to his house to avoid physical contact due to severe obsessive-compulsive disorder and germaphobia. He spends his days tracking his neighbors' movements from his window and scrubbing his environment clean with bleach. He carries immense guilt regarding the death of his baby brother.

Sheila is Matthew's mother, who works at a local salon called Head to Toe. She tries to accommodate Matthew's extreme phobias by providing him with latex gloves and lying to his school about him having mono. Her desperation to find a quick cure for his condition causes tension in her marriage.

Brian is Matthew's father. He refuses to enable Matthew's obsessive-compulsive habits and expresses clear frustration over his son's refusal to leave the house. He attempts to connect with Matthew through normal activities like playing pool but quickly loses patience with his son's limitations.

Melody is a twelve-year-old girl who lives in the neighborhood and struggles to fit in at school. She spends her free time visiting the local cemetery to collect memorial cards left on gravestones. She eagerly joins Matthew in his investigation, acting as his proxy outside the house.

Old Nina is an elderly widow who lives alone in the Rectory, a house that stands out from the others in the cul-de-sac. She rarely leaves her home and keeps a lamp permanently lit in her front window, waiting for the son she lost years ago to return.

Jake is a twelve-year-old boy who acts as the neighborhood bully. As a young child, he suffered from severe allergies that caused a scaly skin condition, leading to intense bullying from his peers. He adopted an aggressive persona to protect himself and now frequently targets his former best friend.

Mr. Charles is an elderly neighbor who takes pride in his garden and his front yard pond. He takes his grandchildren in for a visit, unaware of the chaos that will soon follow. He occasionally attempts to hire Matthew for odd jobs.

Teddy is Mr. Charles's two-year-old grandson. He is a cheerful toddler who carries a favorite blue blanket everywhere he goes. His sudden disappearance from the front yard sets the neighborhood investigation into motion.

Casey is Teddy's older sister. She resents her younger brother and frequently acts out against him when adults are not looking. She is perceptive enough to notice Matthew watching from his window and takes pleasure in mocking him.

Callum is Matthew's baby brother who died shortly after birth several years ago. His memory haunts the Corbin family, and Matthew harbors an intense belief that his own childhood illness caused the infant's death.

Penny is a middle-aged neighbor who lives in the cul-de-sac with her husband Gordon. She previously maintained a close relationship with the Corbin family but has kept her distance from Matthew ever since the death of his baby brother.

Gordon is Penny's husband and a long-time resident of the neighborhood. He typically drives the couple's car and delivers catalogs around the cul-de-sac. He grows increasingly annoyed by Matthew's habit of watching the neighbors.

Dr. Rhodes is a local therapist hired by Matthew's parents. She officially diagnoses Matthew with obsessive-compulsive disorder and introduces the concept of cognitive behavioral therapy to help him face his fears.

Wallpaper Lion is a section of wallpaper in Matthew's room that resembles a lion's face. Because Matthew isolates himself from human contact, he treats the pattern as a living companion, confiding his secrets and fears to it daily.

Tom is a boy from school who used to be Matthew's best friend. He was the first person to notice Matthew making excessive trips to the school bathroom to wash his hands before Matthew stopped attending classes entirely.

Mr. Jenkins is the physical education teacher at Matthew and Jake's school who also happens to live in their neighborhood. He is known for being unnecessarily tough on his students and frequently grimaces at Matthew's house.

Hannah is Mr. Jenkins's wife. Her pregnancy acts as a major trigger for Matthew, whose anxiety heavily revolves around the vulnerability of infants.

Sue is Jake's mother. She raised Jake on her own after her husband left the family. She originally formed a close bond with Matthew's mother when they were pregnant at the same time.

Claudia is Melody's mother. She shares dog-walking duties with her daughter and keeps a close eye on Melody's communications to ensure she stays out of trouble.

Melissa is Teddy and Casey's mother. She leaves her children in the care of their grandfather before traveling, only to rush back when she receives news of Teddy's disappearance.

Officer Campen is a uniformed policeman who conducts the initial neighborhood canvas after Teddy Dawson vanishes. He grows suspicious of Matthew's reclusive behavior and habit of tracking neighbors.