Father Cheroki is a priest at the Leibowitz Abbey. He tends to respect a person's social position, rather than the person who actually occupies it. As such, he hears Francis's confession, but doesn't consider it worth his time.
On the other hand, he is incredibly respectful, even afraid, of Abbot Arkos.
And yes, his name is meant to hint at his Native American heritage. The fact that he comes "of baronial stock from Denver" is meant to be an ironic twist on the story's construction of American history.
Brother Fingo is a sport, or mutant, hailing from Minnesota country. Ugly as all get-out, he "adopt[s] [an] affable manner to compensate." This works, at least with Francis, who is "charmed" by the brother's humor (8.4).
Fingo's importance in the novel is that he works on a woodcarving of Leibowitz. This carving looks suspiciously like a certain pilgrim from Francis's past.
Horner is a kindly, gentle brother. He's also master of the abbey's copyroom, or scriptorium. He lets Francis work on a side project for an hour or so a day, and Francis chooses to create an illuminated copy of the Leibowitz blueprint.
Sarl is one of Francis's fellow copyists. His side project is to work out a "'mathematical method for finding missing words'" on burned pages. After working for five decades, he is able to restore five pages of text. That is, before he slumps over dead at his copying desk.
Jeris is another copyist, one who is "pretentious in his sarcasm" (7.73). Jeris teases Francis about the importance of the Leibowitz blueprint. He takes over as master of the copyroom after Horner.
A thoughtful and gentle monsignor from New Rome, Aguerra is the postulator for the canonization of Leibowitz. That is, he's the guy making the case that Leibowitz should be beatified. As part of his investigation, he reopens the Fallout shelter for investigation. His admiration of Francis's illuminated manuscript allows the young brother to finish the work.
The devil's advocate (advocatus diaboli) sent from New Rome to the abbey. Unlike the postulator, the devil's advocate's job is to make the case against beatification. As such, Flaught is Aguerra's opponent in the case for Leibowitz's sainthood.
He's also the man's opposite in every imaginable way: the ill-tempered and forceful yin to Aguerra's calm and thoughtful yang.
The robber robs people. Bet you didn't see that one coming. Specifically, he robs Francis while the brother is on his way to New Rome. He takes the illuminated manuscript from the monk, believing the illuminated one to be more valuable than the original.
The robber is accompanied by two of the Pope's Children—mutants whose mothers did not have an abortion. When Francis returns to buy back the illuminated manuscript from the robber, these two murder the poor monk before eating him. They leave what remains for the buzzards.
Brother Kornhoer is a brilliant monk who moonlights as an inventor at Leibowitz's Abbey. Brother Armbruster is the librarian and a luddite. If these two were any more of an odd couple, we swear they'd have their own theme song.
When the arc lamp is set up for presentation, Armbruster argues against it. He sees the gizmo as distracting from the work that God has set before them. It is also, arguably, a return of the evils of technology to the world. He even refers to it as witchcraft (14.38).
Particularly vexing for Armbruster is the fact that they remove a crucifix from the wall to hang the lamp.
Kornhoer is the one who invited Than Taddeo to the abbey to study the Memorabilia. Though, in truth, he probably wanted to show off his arc lamp more than call Taddeo to study. Kornhoer sees the arc lamp as necessary for progress and, as he later says, it's a type of prayer for him (22.20).
Although a huge fan of Taddeo and his work, Kornhoer declines an invitation to study at the collegium. He is the one who returns the crucifix to its original position on the wall.
He is the mayor of Texarkana during the novel's second part, Fiat Lux. Although we never meet him on the page, he acts as though he is more of a Caesar than mayor. And like any conquering ruler, he pits Mad Bear's nomadic tribes against the Laredans to carve out a larger, more secure nation for his rule.
Marcus Apollo is the nuncio (papal ambassador) in Hannegan's court. His letter warns Dom Paulo that Thon Taddeo is a "political captive of [Hannegan's] state" (13.12). He later warns New Rome of Hannegan's plans to attack Denver.
For this, he is hanged and cut down while still alive to be drawn, quartered, and finally flayed, granting him a spot on the future's ever-growing list of martyrs.
Marcus Apollo's secretary. He betrays Marcus Apollo but only faces some severely heinous torture for it. (Only.) He manages to escape to the Leibowitz Abbey, bringing news of Hannegan's declaration that the Pope is a traitor and a heretic.
A leader of a violent nomadic tribe. He receives weapons and strategy from Hannegan, but doesn't realize that Texarkana's mayor is using him to destroy his enemies.
Dom Paulo's prior.
During a few press conferences, the defense minster dodges the reporters' questions through some daft political maneuvering. Sorry, we meant to say deft. Although now that we think about it, it's pretty daft too.
Anyway, these press conferences keep the readers up-to-date on the political situation during Fiat Voluntas Tua. So we're grateful for them.
This nameless woman and her child suffer severe radiation sickness and flash burns during a nuclear attack. They become the center of the novel's debate between the Church and the state. Dr. Cors suggests she and the child accept state-sponsored euthanasia.
Father Zerchi argues that they should continue living, and let the abbey care for them. In the end, she decides on euthanasia, despite her religious beliefs.
Pat is Father Zerchi's secretary. He may or may not have voided the warranty on Zerchi's Autoscribe. Lucky for him, the warranty of the entire world is soon voided anyway. Okay, maybe that's not so lucky after all.
Father Zerchi's contact in New Rome. Together, they set up the launch of Quo peregrinatur.
Mrs. Grales's dog.
Father Zerchi's prior.