Arthur is a senior at Rackham College who works at the Brooks Library and hopes to pursue graduate studies at Oxford University. Though he recently inherited a trust fund from his late father, he often feels out of place among his wealthy friends. He possesses a strong sense of responsibility, which leads him into a coercive extortion scheme to protect his incarcerated mother. His moral boundaries are tested when he and his friends accidentally summon a demonic entity.

Gwen is a working-class teenager whose mother serves as the housekeeper for Colin Wren's grandfather. She plans to bypass a traditional four-year college to train as an emergency medical technician, driven by a sincere desire to comfort frightened people. Her practical nature contrasts with the privileged attitudes of Arthur's college friends. She quickly develops a connection with Arthur, though their differing backgrounds create friction.

King Sorrow is a demonic dragon entity originating from a parallel dimension known as the Long Dark. He feeds on human fear and suffering, seeking out vulnerable individuals to exploit for his own amusement. He establishes a coercive agreement with the college students, demanding an annual sacrifice in exchange for his protection. He views humans purely as sustenance and entertainment, lacking any capacity for empathy or negotiation.

Colin is a wealthy, privileged student living on a massive estate called The Briars with his grandfather. Orphaned by a car accident, he has developed a deep interest in computer science, esoterica, and the supernatural. He approaches morality with a detached, utilitarian mindset, readily accepting the arrangement with the demonic entity as a tool to eliminate dangerous individuals from society.

Van is a progressive college student who covers government stories for his father's newspaper. He shares a close but complicated bond with his twin sister, often stepping in to shield her from the consequences of her volatile temper. He relies on humor to hide deeper insecurities and harbors strong feelings for his friend Allie. He often finds his left-leaning political views at odds with his sister's fierce conservatism.

Donna is a fiercely conservative college student who relies heavily on her twin brother for emotional regulation. She harbors deep-seated anger and paranoia stemming from the unsolved childhood abduction of her best friend. Her trauma manifests as hostility toward the world and a quickness to judge others, making her eager to direct their newly summoned entity against people she deems dangerous.

Allie is a college student who finds comfort in statistics, using logic and numbers to brace herself against life's unpredictable tragedies. She carries the weight of past family trauma, including the loss of her brother to cancer. She represses significant aspects of her identity, leading to complicated dynamics within the friend group, particularly regarding her relationships with the McBride twins.

Erin is an Episcopalian pastor and Arthur's mother, currently incarcerated for her involvement in a protest that led to accidental property damage and a death. She accepts full responsibility for the consequences of her actions, demonstrating a strong moral compass. She utilizes her time in prison to lead community-building efforts, such as a book club, hoping to transform her past mistakes into opportunities for positive impact.

Jayne is a highly manipulative extortionist who uses threats of violence to force Arthur into stealing rare books from his college library. She frequently subjects her younger sister to emotional and physical abuse. Driven by greed and malice, she operates with a sense of untouchability, entirely unaware of the supernatural forces Arthur and his friends are about to unleash.

Tana is a pregnant teenager living under the oppressive control of her abusive older sister and incarcerated mother. She forms an unlikely connection with Arthur after his simple act of kindness at the prison. She hopes her impending motherhood will provide an escape from her toxic family environment, often displaying a quiet resilience despite her difficult circumstances.

Robin is a British book editor who encounters the friend group during a highly stressful international flight. She maintains a calm and friendly demeanor in chaotic situations, eventually becoming a trusted confidante to the core group of friends.

Valentine is a commanding officer for a private security firm called Thermopylae. He operates a clandestine industrial facility where he uses psychological pressure and physical isolation to extract information. He specifically targets the friend group to learn more about their connection to the supernatural entity.

Llewellyn is Colin's grandfather and a former military psychologist who served during the Vietnam War. He maintains a private study filled with strange occult items known as the Cabinet of Curiosities. He frequently relies on medical marijuana to manage his health conditions and employs Gwen's mother as his housekeeper.

Elwood Hondo is a murderous ghost originally conjured during a military-funded research study based on the Philip Experiment. The recorded footage of his summoning causes violent poltergeist activity in the present day, introducing the college students to the reality of the supernatural.

Daphne is a harsh, abusive woman serving time at the Black Cricket House of Correction for shooting a postal carrier. She maintains a toxic influence over her daughters even from behind bars and harbors deep hostility toward Erin Oakes, going so far as to frame her with contraband weapons.

Ronnie is Jayne's boyfriend and willing accomplice in her extortion schemes. He assists in intimidating Arthur and enforcing Jayne's demands, acting as muscle to ensure Arthur complies with their orders to steal rare library books.

Cady is Donna McBride's childhood best friend whose unsolved abduction left a lasting mark on Donna's psychology. The unresolved nature of her disappearance fuels Donna's present-day anger and her desire for punitive justice.

Enoch Crane is a deceased occultist and suspected devil worshiper. His notorious journal details his indoctrination of followers and provides the specific instructions for summoning the entity from the Long Dark, serving as the catalyst for the students' supernatural pact.

Horation travels under the legal name Randy Mathers. He holds extreme racist and anarchist views, promoting a dangerous ideology that targets vulnerable people. He believes he is protected by divine forces because of his violent actions against his own pastor.

Svangur is a supernatural troll who resides in the United Kingdom. He possesses deep knowledge of the local cave systems and ancient crypts, offering his services as a guide in exchange for money and modern technology.