Born in 1919 in a train station lavatory, Harry is a kalachakra—a person who is endlessly reborn into the exact same life but with full memories of his previous existences. He is uniquely burdened as a mnemonic, meaning his memory of past lives never fades, leading to severe early psychological struggles and periods of institutionalization. Driven by a desire for purpose and an innate sense of morality, he spends successive lives studying technology, medicine, and warfare to understand his condition and the changing world.

Vincent is a brilliant and charismatic physics student with an insatiable hunger for scientific knowledge. Like Harry, he remembers his past lives perfectly. He views conventional morality as a constraint and argues that those with foreknowledge have a duty to accelerate technological progress, regardless of the human cost. His ultimate goal is to unravel the fundamental laws of the universe.

Virginia is a worldly, well-established member of the Cronus Club, a secret society of people who relive their lives. She acts as a guide to newly awakened kalachakras, taking care of children and explaining the rules of their existence. She adheres strictly to the Club's philosophy of non-interference and warns Harry about the dangers of revealing his exact birth details.

Akinleye is a fellow kalachakra who initially embraces a hedonistic lifestyle to cope with the repetitive nature of her existence. She argues that personal experience and pleasure are the only meaningful pursuits in an unchangeable timeline. Her perspective challenges Harry's sense of duty, offering a contrasting view on how to endure immortality.

Phearson is an American intelligence agent operating during the Cold War. He discovers the existence of people with foreknowledge and views them as strategic assets for geopolitical dominance. Convinced of his own moral superiority, he is willing to use imprisonment and physical torture to extract information about the future.

Jenny is a skilled surgeon and the first person Harry attempts to fully confide in regarding his cyclical lives. As a mortal woman with a linear experience of time, she is terrified by his foreknowledge. Her inability to comprehend his condition leads to a tragic breakdown in their marriage and leaves lasting emotional scars.

Charity is a pragmatic and resourceful member of the Cronus Club dedicated to identifying and extracting young kalachakras from their linear families. She serves as an institutional anchor, providing logistical support and ensuring that members are protected during their vulnerable early years.

Rory is the aristocratic master of Hulne Hall and Harry's biological father. An emotionally distant figure dealing with the decline of his family's estate and his own war injuries, he represents the rigid class system of early 20th-century Britain. He is entirely unaware of his true connection to Harry.

Patrick is the groundskeeper at the Hulne estate and Harry's adoptive father. He is a quiet, stoic, and steadfast working-class man who provides a stable and grounded upbringing. Despite the emotional distance that develops after the loss of his wife, he remains a figure of quiet decency.

Dr. Abel is a psychiatrist who views Harry's claims of living past lives as severe delusions. He aggressively treats Harry with heavy sedatives and experimental antipsychotic drugs, exploiting Harry's vulnerable state to interrogate him about future events.

Christa is a seven-year-old kalachakra who serves as a messenger. She delivers a dire warning passed back through successive generations from a thousand years in the future, setting Harry on his primary quest to investigate the accelerating end of the world.

Elizabeth is a servant at Hulne Hall who becomes pregnant by the master of the estate. Expelled to avoid scandal, she tragically dies shortly after giving birth, leaving an enduring shadow over Harry's origins.

Harriet is the loving adoptive mother who provides Harry with his earliest sense of warmth and normalcy. Her tragic death from cancer profoundly impacts both Harry and his adoptive father, forcing Harry to mature quickly and rely on his hidden knowledge.

Victor is a historical kalachakra who attempted to use his knowledge of the future to alter the course of the Thirty Years War. His well-intentioned interference triggered a devastating chain of events leading to a nuclear apocalypse, making him the central cautionary figure for the Cronus Club's doctrine of non-interference.

Ugly Bill is a cruel and abusive nurse who torments patients under his care. He subjects Harry and others to physical mistreatment, embodying the horrors of the mid-20th-century psychiatric institutional system.

Constance is Rory's mother and the imposing matriarch of the Hulne family. She is fiercely protective of the family's reputation, willingly discarding vulnerable servants to avoid scandal. She later displays heavy condescension toward Harry's academic ambitions.

Daniel is an East German engineer credited with designing a highly advanced, anachronistic portable radio. He is a linear human who unknowingly benefits from accelerated technological leaks and briefly points Harry toward the true source of the scientific anomalies.

Fidel is a mercenary kalachakra who scorns the passive philosophy of the Cronus Club. He spends his recurring lives fighting in various wars solely for the thrill of unpredictability, viewing existence as inherently meaningless but exhilarating.

Richard is a linear human who commits a string of murders across multiple cycles of time. His crimes force Harry to confront the ethical dilemma of preemptive justice, testing whether Harry is willing to intervene and kill a man before he has committed the crimes in a given life.

Rosemary is a woman who becomes close to Harry during one of his lives. Her tragic murder at the hands of Richard Lisle serves as a catalyst for Harry to experiment with direct intervention and revenge.

Anya is an observant member of the Cronus Club who points out a critical pattern in the incoming messages from the future. She deduces that the world's end is happening earlier in each successive cycle, helping Harry realize that the catastrophe is being actively caused by a person.