Tom is an approximately 400-year-old man who ages roughly one year for every fifteen. He currently lives in London working as a high school history teacher. Tormented by headaches that trigger vivid memories of his centuries-long life, he tries to maintain a low profile. He actively searches for his long-lost daughter and struggles with the emotional cost of outliving the people he loves.

Hendrich is the extremely wealthy founder and leader of the Albatross Society. Over seven centuries old, he uses his resources to locate and recruit others with the slow-aging condition. He provides new identities and protection for his members, demanding strict loyalty and obedience in return. He actively discourages personal attachments, viewing regular humans as fleeting.

Rose is an Elizabethan woman who takes Tom in after he arrives in London grieving and destitute. She forms a deep bond with him and accepts his unusual lack of aging. Living in a small cottage in Hackney with her younger sister, she builds a stable life despite widespread illness and societal superstition.

Marion is the daughter of Tom and Rose, born in the early 17th century. She inherits her father's slow-aging condition, placing her in immediate danger from superstitious locals and witchfinders. Her long-standing disappearance fuels Tom's actions for centuries as he desperately uses the society's resources to locate her.

Omai is a Tahitian man who shares the slow-aging condition. Originally revered as a god-like figure in his homeland, he forms a kinship with Tom during Captain Cook's 18th-century voyages. He fully embraces life and chooses to live openly without fear, standing in direct opposition to the restrictive rules of the Albatross Society.

Camille is a French teacher at the London high school where Tom secures employment. She observes Tom's odd behavior and recognizes him from a vintage photograph at a Parisian hotel where she used to work. Determined and inquisitive, she actively seeks to uncover the truth about Tom's past.

Daphne is the modern-day headteacher at the London school. She hires Tom despite his apparent lack of recent experience, challenging him to make history feel alive for the students. She manages the staff dynamics and keeps an eye on the developing connection between Tom and Camille.

Agnes is a long-lived member of the Albatross Society who was once known as the pirate Flora Burn. She acts as a handler and recruiter for Hendrich, tracking down newly discovered individuals with the condition and bringing them to America.

Dr. Hutchinson is a 19th-century medical professional who discovers the slow-aging condition. Tom approaches him hoping to find a cure or information about others with the affliction, but their interactions bring about dangerous consequences.

Anton is a troubled modern-day student in Tom's history class. Initially detached from his coursework and associating with older bullies, he slowly engages with his education after Tom takes a personal interest in his well-being.

Grace is Rose's younger sister. Following the death of their parents and brothers, she relies heavily on Rose. She welcomes Tom into their home and eagerly asks him for lute lessons.

William Manning is an Elizabethan witchfinder who uses fear and false evidence to condemn local women. He targets Tom's mother and repeatedly poses a dire threat to Tom's family throughout the 16th century.

Mary Peters is an enigmatic woman living in 16th-century London who later reappears in Tom's life at a modern-day retirement home in Surbiton. She holds crucial information about Tom's missing family.

Martin is the modern-day music teacher at Tom's London school. He displays a competitive streak and attempts to test Tom's musical knowledge during a staff gathering at a local pub.