Evelyn is a 17-year-old girl currently living in Wales under the name Branwen Blythe. She reincarnates continuously across a thousand years, retaining fragmented memories of past lives where she invariably dies before her eighteenth birthday. In her current life, she races against time to donate stem cells to her sick sister before her deadly birthday arrives. She possesses a deep capacity for hope despite knowing her violent historical pattern.

Arden is Evelyn's eternal companion who inexplicably tracks her down in every timeline. Unlike Evelyn, he appears to remember every detail of their past lives together. A harsh but mysterious protective instinct motivates him to end her life before she turns eighteen in each incarnation. He often writes poetry and guards his emotions closely.

The Mother is a terrifying, witch-like entity with pure white hair and curling black fingernails. She interrupts Evelyn and Arden during their past lives, radiating malice and danger. Her sudden appearances consistently prompt Arden to take drastic, fatal action to protect Evelyn from her presence.

Gracie is Evelyn's 14-year-old sister in her current lifetime in Wales. She has leukemia and urgently requires a stem cell transplant. She balances a sunny disposition with dark humor, giving Evelyn a powerful motivation to survive past her upcoming eighteenth birthday.

Mrs. Blythe is Evelyn's mother in the 2022 Wales timeline. She carries the heavy grief of losing her husband to a drunk driver ten years ago and now faces the terrifying prospect of losing her youngest daughter to cancer. She urges Evelyn to resume psychiatric therapy to manage the family's stress.

Dylan is a farmhand working for the Blythe family in Wales. He maintains an upbeat, highly energetic persona and helps the family with daily manual tasks and transportation to the hospital. He establishes himself as a supportive presence for Gracie during her medical treatments.

Ceri is a young newcomer to the Welsh town who rents an apartment above the local gas station. When he visits the bookstore where Evelyn works, he buys a poetry collection and gives her his phone number. Because of his sudden arrival right before her eighteenth birthday, Evelyn strongly suspects he is the current incarnation of Arden.

Dr. Chiang is a psychiatrist who previously treated Evelyn. Evelyn stopped attending sessions because she found it impossible to explain her past lives safely. Evelyn later seeks her out in hopes of securing a medical favor through Dr. Chiang's wife.

Dr. Schneider is a surgeon and the wife of Dr. Chiang. Evelyn hopes to convince her to bypass normal hospital scheduling and perform the critical bone marrow procedure for Gracie ahead of time.

Léon Cazares is a successful clothing designer currently working in a café in 2054 Scotland. He harbors a vague sense of searching for something important and feels an unexplained, deep attachment to a Welsh actress named Gracie Blythe after she wears one of his red gown designs.