Lib is a 29-year-old English nurse trained by Florence Nightingale during the Crimean War. She arrives in rural Ireland to observe a young girl claiming to survive without food. Highly observant and fiercely professional, Lib is deeply skeptical of Catholic traditions. She relies on scientific principles to maintain emotional distance from her patients, though she carries unspoken grief from her past.

Anna is an 11-year-old Irish girl who claims not to have eaten any food for four months, subsisting only on divine sustenance. She is deeply devout, curious about the natural world, and highly intelligent. Despite her frail physical state, her steadfast faith commands the attention of her family and draws visitors hoping to witness a miracle.

William is a young, educated Irish journalist sent by the Irish Times to investigate the claims surrounding Anna. Having begun his career reporting during the Great Famine, he holds strong anti-English sentiments and a firsthand understanding of starvation. He serves as a cultural guide for Lib, challenging her prejudices.

Sister Michael is a middle-aged Irish nun belonging to the Sisters of Mercy. Like Lib, she is a trained nurse with extensive medical experience, having served during cholera epidemics and the Great Famine. Her deep Catholic faith and vow of obedience strongly influence her approach to observing Anna.

Dr. McBrearty is the Anglo-Irish physician supervising the watch over Anna. He firmly believes in the validity of Anna's fast, hoping she might represent a scientific or spiritual marvel. He frequently dismisses Lib's medical concerns, attributing her warnings to maternal hysteria.

Rosaleen is Anna's mother, living in a dark, rural cabin in Athlone. She deeply believes that her daughter's survival without food is a divine miracle. She frequently clashes with Lib, especially when the nurse imposes strict rules that restrict the family's access to Anna.

Malachy is Anna's father, an Irish farmer who shares his wife's belief in their daughter's miraculous condition. He frequently manages the household's visitors and maintains their small farmyard, initially drawing Lib's suspicion regarding his involvement in Anna's fast.

Pat is Anna's older brother. Initially presumed by Lib to have emigrated from Ireland, he is a significant figure in Anna's life whose history casts a heavy shadow over the O'Donnell household and heavily influences Anna's spiritual anxieties.

Father Thaddeus is the local village priest in Athlone. He acts as a spiritual authority for the O'Donnell family and attempts to explain Catholic customs to a skeptical Lib. He is a frequent visitor to the O'Donnell home, providing religious guidance to Anna.

Kitty is the maid working for the O'Donnell family. She assists with household chores and the maintenance of Catholic folk traditions, such as leaving milk out for the fairies, which frequently puts her at odds with Lib's rigid standards.

Dr. Standish is an English doctor who receives special permission to examine Anna. He is harsh, demanding, and utterly dismissive of the girl's spiritual claims, quickly concluding her situation is merely a case of hysteria.

Mr. Flynn is a member of the local committee established to oversee the watch on Anna. He views the young girl's fasting not as a medical crisis, but as a potential miracle that could bring glory and religious tourism to Ireland.