Isaac Edward Leibowitz
The titular character of the novel, Leibowitz lived long before the events of the novel take place, in what has been approximated to be 600 years. Before the Flame Deluge and Simplification, Leibowitz was a Jewish American employed as an engineer with the American military. Unlike many others, he survived the Flame Deluge nuclear apocalypse. Realizing that there was a large public backlash towards intellectualism, knowledge and books following the Deluge, all of which were blamed for the Flame Deluge, Leibowitz secretly saved and copied books. These acts gather a small and clandestine following, though he is eventually discovered and murdered. After his death, Leibowitz's followers become a religious order named the Albertian Order of Leibowitz. In the period that the novel is set, Leibowitz becomes a candidate a candidate for Catholic Sainthood.
Brother Francis Gerard
Brother Francis Gerard is the protagonist of the novels first section, "Fiat Homo." As the novel begins, Brother Francis is completing a pilgrimage is completing a pilgrimage in the desert. It is revealed that he is 17 years old, a priest in training, and that the story is set some time in the 26th century. In the desert, Francis Gerard comes across an unnamed man who is later revealed to possibly be a Pilgrim named Benjamin. The man had completed drawings in Hebrew on rock faces in the desert, and when Gerard lifts one of the rocks up, he discovers an underground shelter. The shelter is stocked with writings and books once owned by Leibowitz. Brother Francis Gerard brings these relics to his abbey, which causes a large sensation as the monks are members of Leibowitz's order, The Albertian Order of Leibowitz. After Leibowitz is named a saint, Brother Fancis travels to New Rome to represent Saint Leibowitz. Francis eventually meets the Pope, though is slaughtered on his return trip to the Abbey. His body is found by the unnamed man, and the body is buried.
Monseigneur Aguerra
In the 26th century, Aguerra is sent to the desert by the Catholic Church to investigate the case for Leibowitz's sainthood. He is a pious man, and a deep believer in Leibowitz's miracles. He eventually successfully argues for Leibowitz's sainthood.
The Wanderer
The wander, who guides Brother Francis towards Leibowitz's relics is an uncertain character, though an important character nonetheless. It is suggested that he is named Benjamin, though this is not verified. He has lived a very long life and is capable of writing in Hebrew, suggesting some religious education. Some suspect that he is a reincarnated form of Leibowitz, others suspect him to be a relative of Leibowitz. Regardless, no definitive answer is offered for the character of the Wanderer. He eventually finds the murdered body of Brother Francis and buries it in the desert.
The Children of the Pope
Genetically Mutated by the nuclear fallout from the Great Deluge, the Children of the Pope is a term used to describe those that are physically or mentally disabled. It is suggested that they exist in large numbers. A group of them eventually kill Brother Francis as he returns from New Rome.
Dom Apollo
A priest in the novel's second part, "Fiat Lux" Apollo reveals much about the Great Deluge that destroyed much of humanity. Engaging in a conversation with a scientist named Thon Thaddeo, Apollo pontificates spiritually about man's destructive tendencies. His name is potentially related to the Greek God Apollo.
Monseigneur Flaught
Alongside Aguerra, Flaught is tasked with examining the case for Leibowitz's Sainthood. He is employed as the Devil's Advocate to discredit claims of Leibowitz's sainthood, though he is ultimately unsuccessful.
Thon Thaddeo
Thaddeo is a major character in the novel's second part, "Fiat Lux." He is a brilliant and inquisitive scholar credited with helping move society from the post-Deluge Dark Ages, and for this reason comparisons are drawn between Thaddeo and Galileo. He is sent to the Leibowitz Abbey in the desert to study the works of Leibowitz, where he makes many novel discoveries.
Dom Jethras Zechri
Dom Jethras Zechri is the protagonist of the novel's third section, "Fiat Voluntas Tua." When nuclear tensions are arising again in the 38th century, Zechri petitions the Church to activate spaceships which will take survivors from the endangered Earth. The nuclear combat escalates and Zechri is unable to escape. An explosion occurs near the Leibowitz abbey, reducing the site to ruins. He dies before making it onto the spaceship.
Brother Joshua
Another character in the third portion of the novel, Brother Joshua is able to board the spaceship before nuclear warfare consumes the world again.
