Hillalum is a miner from the ancient civilization of Elam. He undertakes the monumental, centuries-long project of climbing the Tower of Babylon to dig into the hard vault of heaven. Along the journey, his practical engineering mindset struggles with the awe and doubt surrounding humanity's attempt to reach the seat of God.

Leon Greco is a Boston-based freelance digital designer recovering from severe brain damage. Following treatments with an experimental drug called Hormone K, he begins to develop profound superhuman intelligence. As his cognitive and motor abilities expand, he grows increasingly isolated, viewing himself as superior and utilizing his talents for entirely self-serving purposes.

Reynolds is another recipient of the Hormone K treatment who began his doses slightly before Leon. He possesses equal or greater superhuman intelligence but applies his cognitive powers toward the grand goal of saving humanity, placing him on a direct collision course with Leon's selfish ideology.

Renee is a brilliant mathematics professor who received her doctorate at age twenty-three. She has always defined her life and reality through the strict, beautiful order of mathematical precision. When she discovers a formalism that upends her entire scientific belief system, she falls into a deep depression, feeling her existence has lost its core meaning.

Carl is a biologist and Renee's husband. He attempts to support her through her severe depression but finds himself completely unable to comprehend the mathematical crisis that triggered it. His inability to empathize with her loss of faith in science creates a widening rift in their marriage.

Dr. Louise Banks is a dedicated, highly respected linguist tasked by the military to communicate with newly arrived extraterrestrials called heptapods. Through the arduous process of deciphering their non-linear, two-dimensional written language, she experiences a profound cognitive shift that alters her perception of time, memory, and her relationship with her daughter.

Dr. Gary Donnelly is a physicist assigned to understand the scientific and mathematical concepts of the alien visitors. He works closely alongside Louise, exploring how the heptapods' understanding of physics drastically differs from humanity's, while also developing a personal connection with her over dinners and shared breakthroughs.

Robert Stratton is a Victorian scientist and designer of automata at the Coade Manufactory. Working in the field of nomenclature, he uses seventy-two-letter Hebrew names to animate matter. He dreams of creating cheap, self-replicating automata to assist the working class, but his socialist ideals meet a stark challenge when he joins an elite project concerning human reproduction.

Lord Fieldhurst is a famous zoologist and President of the Royal Society. Representing the conservative aristocratic elite of an alternate Victorian era, he seeks to use nomenclature to control human reproduction, aiming to preserve wealthy bloodlines while preventing the poor from having large families.

Neil Fisk is a man born with a congenital leg deformity who harbors a deep resentment toward God. Following the sudden death of his devout wife during a destructive angel visitation, he desperately searches for a loophole to enter Heaven, knowing he cannot genuinely conjure the unconditional faith required for salvation.

Tamera is a first-year college student who previously attended a high school where calliagnosia, a neural modification blocking the perception of physical beauty, was compulsory. She experiences the complex social politics of physical appearance, eventually disabling her calli to explore how beauty influences relationships and self-worth.

Nanni is an Elamite miner who joins the expedition up the Tower of Babylon. He shares the physical exhaustion and isolation of the climb, learning local lore from other workers as the group ascends past the sun and the moon.

Lugatum is a puller stationed at the lower levels of the Tower of Babylon. He envies the incoming miners because their profession allows them to travel all the way to the top of the structure, whereas pullers remain restricted to specific elevations.

Dr. Shea is the head neurologist overseeing Leon's remarkable recovery. Fascinated by the sudden spike in his patient's intellect, he continues to administer Hormone K, unaware of the extreme superhuman capabilities Leon is secretly developing.

Colonel Weber is the military officer in charge of the human-alien communication project. He remains consistently cautious about protecting human technology and intelligence, prioritizing national security while facilitating the necessary scientific research.

Dr. Ashbourne is Robert Stratton's former lecturer. He confirms the looming crisis of human infertility and experiments with impressing euonyms onto unfertilized frog ova, actively assisting Lord Fieldhurst's secretive genetic agenda.

Benjamin Roth is a Kabbalist mystic who views the science of nomenclature as a holy ritual rather than an industrial tool. He seeks Robert's findings so his brethren can meditate on advanced language, bringing them closer to God.

Sarah is Neil's deeply beloved wife. A devout believer, her soul ascends to Heaven after she perishes during the violent visitation of the angel Nathanael, leaving her non-believing husband desperate to reunite with her across the cosmic divide.

Janice Reilly was born without lower limbs following an angelic visitation. She initially finds purpose in preaching acceptance to disabled audiences, but she experiences profound guilt and confusion when a second visitation miraculously grants her new legs, prompting her to rethink her spiritual message.

Ethan Mead is a married librarian who has spent his life certain that God has a special plan for him. After witnessing the same visitation that cures Janice, he deduces that his purpose lies in meeting her, prompting him to follow her on a pilgrimage.

Maria is a third-year student at Pembleton College and the president of Students for Equality Everywhere (SEE). She passionately campaigns to make calliagnosia compulsory on campus, arguing that it forces people to look beyond superficial appearances and levels the social playing field.