Lenni is a 17-year-old Swedish girl living with a terminal condition in the May Ward of the Princess Royal Hospital in Glasgow. Lonely but fiercely independent and mischievous, she struggles to relate to healthy teens but deeply craves connection. She frequently wanders the hospital corridors, asking probing questions of the staff. To leave a mark on the world, she initiates a project to paint 100 pictures representing her and Margot's combined lifetimes.

Margot is an 83-year-old Scottish woman hospitalized for a heart condition. Having lived a long life filled with intense love, tragic loss, and societal constraints, she eagerly joins Lenni in painting their shared century of memories. She is loving and insightful, carrying a rebellious streak beneath her gentle, elderly exterior. She spends much of her time reflecting on her complex past romances in London and Warwickshire.

Father Arthur is the 60-year-old chaplain at Princess Royal Hospital. Patient, wise, and supportive, he does not pretend to have all the answers to Lenni's probing questions about religion and mortality. The chapel becomes a sanctuary for Lenni thanks to his non-judgmental presence, and he deeply appreciates the life and energy she brings to his final months before retirement.

Meena is a free-spirited, unpredictable woman Margot meets in London. Born Amelia Catherine Houghton, she renames herself to match her fiercely independent lifestyle. She works as a typist while participating in animal rights activism and anti-war protests. Her presence deeply challenges Margot to shed traditional gender norms and societal expectations.

Humphrey is an eccentric, cheerful astronomer and Margot's second husband. He lives in a cluttered farmhouse, keeps chickens named after old Hollywood stars, and helps Margot find light in the darkness following a period of intense heartbreak. He provides a calm, accepting presence in Margot's life, never pressuring her to be anything other than herself.

Lenni's mother is a Swedish woman who wrestles with severe mental health struggles. Her inability to cope with her condition eventually leads her to divorce Lenni's father and return to Sweden alone, leaving a teenage Lenni with unresolved feelings of abandonment.

Lenni's father tries to remain positive for his daughter but struggles profoundly with her terminal diagnosis. The pain of watching her health decline heavily impacts his ability to be present for her in the hospital, leading Lenni to make a difficult decision regarding his visits.

Johnathan Edward Docherty, known as Johnny, is a former glassworks apprentice and Margot's first husband. While initially charming and offering Margot an escape from her difficult home life, he struggles under the pressures of adulthood and family tragedy, often allowing his domineering mother to dictate their lives.

The Temp is an administrative assistant with an art degree who works tirelessly to establish the hospital's art therapy program. After her boss takes credit for her hard work and fires her, she forms a brief but highly impactful connection with Lenni.

Known primarily as "New Nurse," she is a red-haired medical professional recently assigned to the May Ward. Initially insecure about her duties, she breaks through strict professional boundaries to form a genuine, caring bond with Lenni, often sharing candy and personal stories.

Pippa is the hospital's art teacher. Well-intentioned but sometimes awkward around blunt conversations about mortality, she helps facilitate Lenni and Margot's ambitious 100-painting project. She eventually enlists other students to assist when the deadline becomes pressing.

Jacky is the strict, waspish head nurse of the May Ward. She operates as a stickler for hospital protocol, often clashing with Lenni's rebellious nature and refusing to grant her special privileges despite her terminal status.

Mr. Eklund is an unhoused Swedish patient who came to England looking for his missing daughter. Believing he is nearing the end of his life, he offers a frightened Lenni unconventional advice, comparing death to a peaceful sleep.

David George, affectionately known as Davey, is Margot and Johnny's infant son. Born with a heart condition, his brief life profoundly impacts Margot's view of love and loss, becoming a central figure in the stories she shares with Lenni.

Margot's mother is a woman worn down by the stress of World War II and her husband's resulting trauma. She relies heavily on young Margot for emotional support during the family's difficult years, drawing immense joy from Margot's eventual engagement to Johnny.

Margot's father is a World War II veteran who returns home fundamentally changed by combat stress reaction (PTSD). His condition forces him into voluntary treatment centers and creates a lasting emotional distance within his family.

The Professor is Meena's academic employer and romantic partner. He is an enigmatic figure whose presence introduces deep complications into Meena and Margot's lives, frequently prioritizing his own reputation over the activist causes Meena champions.