A 30-year-old Jewish doctor who grew up in upstate New York and attended medical school after a period in England as a Rhodes Scholar. He struggles to articulate his overwhelming compassion and exhaustion during his internship year. To manage the trauma of the hospital, he turns to heavy drinking and emotional detachment. His father is a dentist whose own medical dreams were thwarted, leaving Roy without a strong personal mentor in the field.

A resident supervisor at the House of God who takes an unorthodox, hands-off approach to patient care. He maintains an obsession with an imaginary anal mirror invention and strongly advocates against performing procedures unless medically necessary. Beneath his intense absurdity and callous-sounding rules lies a deep protective instinct for his patients and the interns under his guidance.

Roy's serious girlfriend and a clinical psychologist. She balances the hospital's intense focus on the physical body by emphasizing mental well-being. She attempts to help Roy process his trauma, though she grows increasingly frustrated by his emotional distance and reliance on unhealthy coping mechanisms.

A Black medical intern from Memphis who works closely with Roy during their first rotation. He practices medicine intuitively and projects a carefree demeanor that masks an intensely caring nature. He secured his educational and professional placements through unprompted postcard offers in the mail.

A resident supervisor who serves as a direct contrast to The Fat Man. She is a tense, ambitious workaholic who takes a highly aggressive approach to medical care, ordering extensive tests to chase obscure diagnoses. She struggles to forge an identity outside of her career, leaving her personal life neglected and her apartment unpacked.

A police officer who frequently spends time in the emergency room at the House of God. He and his partner issue antiquated statements and demonstrate a surprising awareness of the hospital staff's personal lives. He holds a latent interest in philosophy and psychology that belies his profession.

A police officer and partner to Gilheeny. He spends portions of his shift in the emergency room observing the medical staff and discussing psychology with the hospital's psychiatrist. He masks deep intellectual curiosity behind his badge and uniform.

An attractive nurse at the House of God. Educated by Catholic nuns, she carries a lingering sense of guilt but embraces her physical vitality within the hospital's atmosphere of illness and death. She provides a distraction for the doctors coping with their harrowing environment.

An intensely fit ICU doctor and marathon runner. He is obsessed with cardiac health and convinced that hobbies like fishing and running are crucial for mitigating the stress of medical work. He brings his own heart-healthy meals to the hospital rather than eating cafeteria food.

A medical intern whose wife is also an intern at a different hospital. He becomes deeply overworked, isolated, and anxious under the intense pressure of the internship year. Jo's aggressive micromanagement of his cases accelerates his physical and mental decline.

One of Roy's fellow medical interns at the House of God. He struggles heavily with the demands of the job and harbors long-term resentment toward his parents, which eventually boils over into an explosive phone argument.

A fellow medical intern whose personal life suffers due to the hospital's demanding schedule. He develops a dark fascination with performing postmortem autopsies and invasive procedures as a way to cope with his environment.

The Chief of Medicine at the House of God. He represents the administrative, bureaucratic side of the hospital system, viewing patients more as numbers than individuals. He remains largely baffled by the emotional struggles his interns face and focuses heavily on retaining them for subsequent years.

A senior doctor and higher-up in the hospital administration. He wields significant influence over the careers of the younger doctors, controlling access to prestigious fellowships and demanding compliance with hospital bureaucracy.

A psychiatrist at the House of God. He possesses a calm and curious demeanor that stands in direct contrast to the frantic pace of the medical doctors. His balanced schedule and approach to the human mind earn him the awe of the local police officers.

An elderly patient who frequently goes in and out of the hospital. She suffers from dementia and becomes a focal point for the conflicting medical philosophies of The Fat Man, who prefers minimal intervention, and Jo, who orders extensive and painful tests.

A man in his fifties and a former physician at the House of God. He is admitted as a patient with what he suspects is terminal cancer. He remains encouraging and empathetic toward the overwhelmed interns, remembering his own time in their shoes.

A nurse at the House of God who begins dating The Runt. She becomes part of the interns' social circle as they find ways to handle their grueling schedules and emotional burnout.