Chief Bromden is a tall, half-Native American man with paranoid schizophrenia who has been the ward's longest-standing patient for about ten years. He presents himself to everyone as deaf and mute as a survival mechanism. He frequently experiences hallucinations of a thick fog and pervasive machinery he calls The Combine, which he believes controls society and enforces strict conformity. Despite his physical size, he feels entirely powerless against institutional authority.

McMurphy is a large, redheaded veteran transferred to the psychiatric ward after pleading insanity to escape manual labor at a work farm. He is a boisterous gambler fond of fighting, openly mocking the hospital's oppressive rules. His arrival immediately upsets the carefully controlled balance of the ward as his natural instincts stand in direct opposition to institutional expectations.

Nurse Ratched, also known as the Big Nurse, is the strict head nurse of the psychiatric ward. A former army nurse, she maintains absolute control over the patients and staff through psychological manipulation, intimidation, and an impeccably enforced daily schedule. She weaponizes group therapy sessions to encourage patients to attack one another's vulnerabilities.

Billy is an unmarried man in his thirties who speaks with a severe stutter and suffers from paralyzing anxiety. He is a voluntary patient who remains deeply infantilized, lacking the confidence to face the outside world or assert independence from the women in his life.

Harding is a highly educated, articulate patient who initially acts as the informal leader of the acute patients. He is insecure about his marriage to his glamorous wife and hides in the hospital voluntarily because he fears the judgment of the outside world.

Dr. Spivey is the official physician of the psychiatric ward. Though he technically holds authority over the nursing staff, he is a submissive man who typically defers to Nurse Ratched's judgments. He develops a fast rapport with McMurphy, which occasionally emboldens him to support patient initiatives like organizing a separate game room.

Cheswick is an acute patient characterized by his sudden, explosive outbursts of frustration that quickly fizzle into hesitation. He is often the first to voice support for challenges to the ward's rules, desperately seeking a stronger leader to champion the patients' grievances.

Candy is a carefree sex worker and a friend of McMurphy's from outside the hospital. She accompanies the patients on their fishing excursion, bringing a dose of uninhibited energy that starkly contrasts with the sterile femininity represented by Nurse Ratched.

George is a shy patient who suffers from an intense phobia of germs and dirt, earning him the nickname Rub-a-Dub George. A former fisherman, his practical knowledge of the sea makes him an essential asset when McMurphy organizes an outing on the open water.

Sefelt is an epileptic patient on the ward. Because he despises the severe physical side effects of his anti-seizure medication—specifically rotting gums—he secretly gives his pills away, risking dangerous seizures rather than suffering the pharmaceutical toll on his body.

Fredrickson is a patient who also suffers from epilepsy. Unlike Sefelt, he is terrified of having seizures, so he secretly consumes Sefelt's discarded medication in addition to his own, willing to endure extreme side effects for a sense of safety.

Martini is a deeply hallucinating patient who frequently interacts with objects and people only he can see. Despite his profound disconnect from reality, he cheerfully attempts to include the other men in his vivid imaginary scenarios.

Pete is a chronic patient who sustained brain damage at birth. He generally avoids interaction, spending his days stating how tired he is, but occasionally displays moments of profound clarity about his existence compared to the other men.

Mr. Turkle is an older nighttime aide at the hospital. Unlike the daytime staff carefully selected by Nurse Ratched for their cruelty, Turkle is relaxed, prone to drinking on the job, and willing to bend the rules for the patients in exchange for illicit substances.

Sandy is a friend of Candy Starr's who joins the men for a late-night gathering on the ward. She is friendly, easily amused, and brings an air of outside normalcy to the highly regulated hospital environment.

Ellis is a chronic patient who was originally admitted as an acute case but suffered severe cognitive damage from electroshock therapy. He spends his days immobilized, pinned to the wall in a manner that evokes a crucifixion.

Ruckly is a chronic patient who entered the hospital as an acute case. A lobotomy gone wrong left him permanently impaired, reducing him to staring blankly and occasionally holding an old photograph.

Blastic is an elderly chronic patient on the ward. He features heavily in one of Bromden's terrifying nighttime visions regarding the hospital's mechanical processing of patients, representing the physical deterioration of the institutionalized men.

Scanlon is an acute patient with a penchant for explosives and destruction. He is one of the men who accompanies McMurphy on the ocean fishing trip, finding a renewed sense of confidence outside the hospital walls.

Bromden's father is the former chief of his Native American tribe. He married a white woman and took her last name, and his eventual submission to government forces serves as a core memory for Bromden regarding the destruction of natural strength.

Bromden's mother is a white woman who married a tribal chief. She actively engaged with government appraisers, negotiating the sale of tribal lands and slowly wearing down her husband's resistance to modern bureaucratic forces.

Harding's wife is an attractive woman who occasionally visits him at the psychiatric ward. Their interactions are strained, lacking affection, and marked by her complaints about his friends and his defensive corrections of her grammar.

Billy's mother is a domineering figure in his life who works as a receptionist in the hospital. She speaks to her adult son as if he is still a young boy, dismissing his hopes for marriage and stifling his progression into adulthood.