Johnny is a twenty-something tattoo shop apprentice living in Los Angeles. He spends much of his time drinking heavily and seeking the attention of women, often fabricating stories about his past. After discovering Zampanò’s manuscript, he becomes intensely obsessed with annotating it. This fixation causes his physical and mental health to deteriorate as he isolates himself in his apartment.

Zampanò is an elderly, blind man who lives alone and writes obsessively. He hires various women to read to him and assist in transcribing his extensive analysis of a documentary film that may not actually exist. He barricades himself inside his room, writing the very manuscript that eventually consumes Johnny's life.

Will is a Pulitzer Prize-winning photojournalist in his early forties. His career often requires him to maintain a physical and emotional distance from his family. He moves to the house on Ash Tree Lane hoping to repair his relationship with his partner, but his obsessive drive to document the physically impossible dimensions of their new home strains their connection further.

Karen is a 37-year-old former model and full-time mother. She moves to the Virginia countryside hoping to stabilize her relationship with Will, though she struggles with his frequent absences. When the house begins exhibiting strange spatial anomalies, she develops severe claustrophobia and actively avoids the growing hallway.

Tom is Will’s twin brother, an affable and overweight house builder. He frequently drinks alcohol and uses marijuana, presenting a more laid-back demeanor than his ambitious brother. Despite their long estrangement, he answers Will's call for help and becomes directly involved in exploring the anomalies of the house.

Lude is a hair stylist and Johnny’s close friend. He spends his free time pursuing women and using drugs, serving as the catalyst for Johnny's social outings. He has little interest in intellectual pursuits, preferring a fast-paced hedonistic lifestyle.

Thumper is a stripper and a mother to a three-year-old child. She frequently visits the tattoo parlor, becoming the intense focus of Johnny's romantic and sexual fixation. She patiently listens to his stories and tries to encourage him to leave his apartment when his mental state begins to decline.

Pelafina is Johnny’s mother, who resides at the Whalestoe Institute, a psychiatric hospital. Her relationship with her son is documented through a series of complex, increasingly erratic letters she sends him. She struggles with severe mental illness that fundamentally disrupts her ability to parent Johnny, leading to his placement in the foster care system.

Billy is an engineering professor and a paraplegic who uses a wheelchair. He is a gruff, practical man who approaches the impossible geometry of the house with a scientific mindset. As a close friend of Will's, he brings a rational perspective to the increasingly irrational events occurring on Ash Tree Lane.

Holloway is a professional hunter and explorer brought in to explore the house's ever-shifting interior. He is an intimidating figure with a history of depression and a profound need to conquer the unknown. His obsession with locating the source of a mysterious growl leads him deeper into the dark corridors.

Jed is an employee of Holloway Roberts who accompanies him on the deep exploration into the hallway. He assists with carrying supplies, anchoring the fishing line, and moving through the shifting architecture of the massive spiral staircase.

Chad is Will and Karen's eight-year-old son. He responds to the growing tension in the house by acting out at school and drawing disturbing pictures of black squares and strange creatures. As the house's anomalies increase, he begins spending more time wandering outside.

Daisy is Will and Karen's five-year-old daughter. She initially treats the spatial anomalies of the house as a game, making echoes down the dark hallway. Like her brother, she begins drawing dark, disturbing pictures as the environment in the house grows more hostile.

Wax is a 26-year-old climbing prodigy employed by Holloway Roberts. He joins the expedition into the dark hallway to help climb through its extreme vertical drops and shifting dimensions. He shares a brief romantic moment with Karen before descending into the depths of the house.

Raymond is Johnny's strict foster father. He maintains rigid control over his household, demanding strict obedience and punishing Johnny for his misbehavior at school. Their relationship is marked by discipline and underlying tension.

Kyrie is a German friend of Lude's who interacts with Johnny. She drives him around Los Angeles and shares a brief physical encounter with him. She is involved with a dangerous man known as Gdansk Man, adding an element of threat to her social circle.

Donnie is Johnny’s father, a former pilot from Vermont who later becomes a truck driver after losing his aviation license. His early death in an auto accident leaves a profound impact on Johnny and contributes to the destabilization of his family.

Gdansk Man is Kyrie's boyfriend, an aggressive and physically imposing figure in Johnny's periphery. His presence in Los Angeles introduces an element of physical danger to Johnny's increasingly paranoid state.

Fowler is an actor residing in New York. He engages in an affair with Karen prior to her relocation to Ash Tree Lane, serving as an ongoing temptation and a symptom of her strained relationship with Will.