Jack is a thirteen-year-old boy struggling to adjust to a new life in Maine following the sudden death of his mother. Raised in landlocked Kansas, he feels alienated by the ocean and isolated at his new boarding school. He possesses a strong foundational knowledge of stars and constellations, taught to him by his father. Grieving and angry, he tags along on a sprawling wilderness quest that challenges his understanding of loyalty.

Early is a thirteen-year-old student at Morton Hill Academy who lives in a basement workshop instead of the dorms. Having lost both parents, he sets his own schedule, attending only the classes he chooses and listening to specific musicians based on the weather. He views the mathematical constant pi as a narrative story, reading colors and words within the digits that lead him on an obsessive search for his brother.

John is a captain in the Navy who recently returned from fighting in the European Theater of World War II. Accustomed to military stoicism, he communicates sparingly and struggles to comfort his grieving son. He relocates Jack to a boarding school near the Portsmouth Naval Shipyard, attempting to bring order to their fractured lives through action rather than emotional discussions.

MacScott is a gruff, scarred woodsman with a black eye patch who runs a logging barge. He operates out of the Bear Knuckle Inn and is obsessively tracking a massive black bear along the Appalachian Trail to collect a bounty. Ruthless and heavily armed, he poses a direct physical threat to the boys when their paths intersect in the wilderness.

Fisher is Early's older brother and a legendary athlete at Morton Hill Academy, famed for his rowing and football achievements. Following graduation, he enlisted in the military to fight in World War II. Official reports indicate his entire squad was killed in France, but Early relies on obscure clues to deduce his survival.

Elaine is Jack's late mother, who passed away suddenly from an aneurysm before the main events of the story. She encouraged Jack to appreciate the beauty of the stars rather than just their navigational utility. Her chipped teacup with red flowers remains a poignant symbol of her absence in Jack's life.

Gunnar is a large, bald woodsman originally from Oslo who lives in an isolated log house. Previously a dockworker and paid bare-knuckle fighter in Portland, he fled to the woods after a tragic accident in the ring. He acts as an outfitter and teaches the boys vital survival skills, though he carries deep physical and emotional scars from his past.

Emmaline is a librarian from Portland who introduced Gunnar to poetry, specifically the work of Gerard Manley Hopkins. She encouraged him to quit his dangerous fighting career and pursue higher education, though a tragic turn of events separated them.

Eustasia is a one-hundred-year-old woman living alone in the wilderness. She steadfastly refuses to believe her son is dead and continues preparing his meals every night. When the boys arrive, she incorporates them into her hopeful delusion, providing a strange but comforting respite.

Martin is Eustasia's son who went out decades ago to test a newly engraved rifle and never returned. His disappearance anchors his mother's existence, forcing her to remain alive in a constant state of waiting.

Mr. Blane is a math teacher and rowing instructor at Morton Hill Academy. He introduces the concept of the number pi as the "holy grail" of mathematics, discussing theoretical limits that inadvertently trigger Early's dramatic departure from the classroom.

Miss B. is the librarian at Morton Hill Academy. She maintains the trophy case containing photographs of former students and provides Jack with a quiet refuge during his lonely early days at the school.

Ethel is a thin, overworked barmaid at the wilderness tavern frequented by MacScott and his logging crew. Early interacts with her kindly during the boys' dangerous encounter with the hunters.

Pi, originally named Polaris, is the legendary protagonist of the story Early reads within the numbers on his chalkboard. He is a young navigator who sets out on a worldwide sailing journey to answer life's questions, facing natural disasters, pirates, and profound grief along the way.

Darius is a pirate captain within Early's narrative of pi. He keeps Pi alive aboard his ship solely to listen to the boy's captivating tales of global adventure.

Pauline is a character in Early's story who suffers from a curse placed by a witch doctor. She appears haggard and homely because she takes on the physical appearance of how others describe her, until someone sees her differently.

Mr. Benedict is a fight promoter and Gunnar's former employer on the Portland docks. He exploited Gunnar's massive size for gambling profits and viciously turned on him when Gunnar tried to quit the fighting ring.