Dr. Ryland Grace is a junior high school science teacher and former molecular biologist from San Francisco. He lives a relatively isolated life on Earth, preferring the company of his middle school students over the competitive academic community that previously ostracized him for his unorthodox theories on extraterrestrial biology. After the discovery of the sun-eating Astrophage, his unique scientific expertise draws him into the international Petrova Taskforce. He relies heavily on sarcasm and rapid scientific calculations to manage his intense anxiety about the fate of the human race.

Rocky is an Eridian engineer and the sole surviving crew member of an extraterrestrial spacecraft. He resembles a dog-sized, five-legged spider made of a hard, stone-like mineral, and he requires an extremely hot, ammonia-rich atmosphere to survive. Lacking eyes, he senses his environment through a highly advanced biological sonar system and communicates via musical chords. He possesses incredible engineering skills and approaches scientific problems with meticulous, trial-and-error pragmatism.

Eva Stratt is the leader of the Petrova Taskforce and a former official with the European Space Agency. The United Nations grants her total global authority and absolute legal immunity to solve the Astrophage crisis by any means necessary. She operates with aggressive utilitarian logic, viewing every human life, resource, and ethical norm as expendable if it increases the chances of saving the species. Her background in history heavily influences her fear of the famine and war that will result from the dimming sun.

Yáo Li-Jie is a Chinese astronaut chosen to command the Hail Mary interstellar mission. He approaches his duties with a strict sense of military discipline and personal honor, taking the massive psychological burden of leading a one-way trip incredibly seriously. He maintains a stoic exterior to provide stability for his crewmates.

Olesya Ilyukhina is the primary Russian engineer assigned to the Hail Mary. She brings a boisterous, irreverent energy to the team, frequently using humor and rowdy behavior to cut through the intense anxiety of the project. Underneath her cheerful exterior, she is a remarkably skilled technician capable of maintaining the complex systems of the spacecraft.

Martin DuBois is an American scientist appointed as the primary science specialist for the mission to Tau Ceti. He approaches scientific inquiry with extreme meticulousness, preferring heavily regulated protocols over speculative experimentation. He possesses a rare genetic marker that makes him resistant to the negative effects of long-term medical comas.

Dr. Annie Shapiro is the backup science specialist for Project Hail Mary. She holds a reputation in the academic community as a highly successful DNA researcher, bringing top-tier biological expertise to the mission parameters. Like DuBois, she possesses the necessary coma-resistant genetic marker required for the interstellar journey.

Dr. Lokken is a Norwegian climatologist drafted into the Petrova Taskforce. She strongly objects to Grace's unorthodox theories regarding non-water-based organisms, preferring conventional scientific models. Despite her initial role in climatology, her pragmatic intellect leads to her designing the Hail Mary's transforming centrifuge structure to simulate Earth gravity for the onboard laboratory.

Dmitri Komorov is a Russian scientist responsible for designing the spin drive engine that will propel the Hail Mary. He figures out how to harness the Astrophage's mass-conversion properties to generate immense thrust. He focuses intensely on the extreme energy outputs of his engines, showing very little concern for the highly dangerous forces he manipulates in close proximity to the rest of the crew.

Dr. Lamai is a Thai medical scientist who developed experimental coma-induction technology. Originally intended to help chemotherapy patients sleep through the worst of their treatments, her research is co-opted by the taskforce to keep the astronauts in suspended animation for the four-year trip to Tau Ceti.

Robert Redell is a brilliant engineer serving a prison sentence for embezzlement and his role in a fatal solar farm accident. Stratt temporarily releases him so he can design a massive, continent-spanning blackpanel solar energy conductor to breed the massive quantities of Astrophage required for the mission.

Steve Hatch is a professor at the University of British Columbia tasked with designing the Beetle probes, four small, automated spacecraft intended to return the Hail Mary's findings to Earth. He maintains a shockingly bright outlook on the global crisis, viewing the Astrophage not as a plague, but as a divine gift of clean energy for humanity's future.

François Leclerc is a renowned climatologist who has spent his career fighting to protect Earth's ecosystems. He is forced into a devastating position when Stratt orders him to devise a method to intentionally accelerate global warming, specifically by nuking the Antarctic ice shelf, to buy humanity a few extra decades of warmth as the sun dims.

Adrian is an Eridian and the lifelong mate of Rocky. She remains on their home planet of Erid, which is slowly suffering the atmospheric effects of the Astrophage infestation. Her memory provides Rocky with immense emotional strength and motivation during his isolation.

Marissa is a government scientist and a friend of Dr. Ryland Grace. She is the first person to contextualize the severity of the solar anomalies for him, revealing that the sun is dimming at an exponential rate and setting the stage for the global panic that follows.