Francis Gerard is a seventeen-year-old novice monk holding a Lenten fast in the Utah desert. Belonging to the Albertian Order of Leibowitz, he endures a grueling vigil in hopes of taking his vows. His discovery of an ancient fallout shelter containing technological artifacts challenges his strict obedience and draws unwanted attention from his superiors.

The Old Wanderer is an enigmatic pilgrim traveling the post-apocalyptic desert. He leaves markings in ancient Hebrew and directs Francis toward a hidden structure. His presence sparks persistent rumors regarding his true identity and lifespan across different eras.

Thon Taddeo Pfarentrott is a renowned secular scholar living during a period of renewed enlightenment. He travels to Leibowitz Abbey to study their preserved scientific texts. He represents the returning age of secular science and academic ambition, bringing him into conflict with the religious order protecting the documents.

Abbot Zerchi is the leader of Leibowitz Abbey during an era of advanced technology and looming nuclear warfare. He firmly believes in enduring suffering as part of a divine plan. He oversees a covert operation to ensure the Church's survival off-world while opposing government-sanctioned euthanasia camps on Earth.

Dom Paulo is the abbot of Leibowitz Abbey during a period of political tension and intellectual rediscovery. He attempts to protect the monastery's knowledge while managing the demands of secular scholars and warlords. He suffers from a secret medical condition that causes severe internal hemorrhaging.

Abbot Arkos is the stern leader of the abbey during Francis's time. He is deeply concerned with the political optics of canonizing Leibowitz. He responds aggressively to unverified miracles out of fear they will attract the wrath of anti-intellectual mobs.

Father Cheroki is a priest at the abbey who handles the spiritual needs of the novices. He treats confessions as routine but shows genuine concern for Francis's mental state when the novice reports supernatural encounters.

Brother Fingo is a cheerful, physically unattractive monk who works as an artisan. He is the first person to independently verify the existence of the fallout shelter's entrance, providing Francis with much-needed validation.

Brother Kornhoer is an inventive monk who successfully builds a working electrical dynamo using ancient theories. His practical success challenges the secular scholars who believe scientific progress belongs only outside the monastery.

The Poet is an eccentric resident of the abbey who values fiction and literature over hard science. He frequently uses a glass eye to make metaphorical points and openly mocks the self-serious attitudes of visiting scholars.

Brother Joshua is a monk living in the advanced technological age. He faces a terrifying choice when asked to leave Earth aboard a starship to preserve the Church's teachings elsewhere during an escalating global crisis.

Mrs. Grales is an elderly woman bearing physical mutations from historical radiation, including a dormant second head she calls Rachel. She seeks religious comfort and understanding regarding her condition from the local priests.

Doctor Cors is a government doctor operating a Mercy Camp near the abbey. He firmly believes in providing euthanasia to radiation victims to spare them prolonged pain, putting him in direct ideological conflict with the monks.

Hannegan II is the ambitious mayor of Texarkana. He seeks to conquer and unite the surrounding territories, utilizing backroom deals, warfare, and secular scientific advancements to secure his political power.

Marcus Apollo is the Pope's ambassador to Texarkana. He attempts to gather intelligence on local warlords while maintaining a fragile diplomatic balance in a hostile court environment.

Father Gault is a priest at the abbey and the likely successor to Dom Paulo. He helps manage the day-to-day affairs and engages in theological debates with visiting scholars.

Father Lehy is a priest working alongside Abbot Zerchi during the tense buildup to global war. He assists in managing the abbey's grim reality during the modern era.