Isaiah is a highly intelligent, blue-furred mouse who escapes from an animal testing facility but gets separated from his family. Forced to survive alone in a suburban neighborhood, he uses his unique abilities to evade predators and secure food. He possesses the ability to read, type, and speak human words. He deeply misses his siblings and searches for a way to rescue them from the laboratory.

Hailey is a 12-year-old girl with white hair and pale blue eyes who recently moved to the neighborhood. She faces bullying at school, leaving her feeling isolated from her peers. She keeps a dog named Dolce and a cat named Red Boy. Her open-mindedness allows her to form an unusual bond with Isaiah after he sneaks into her house.

Mikayla is a brown mouse belonging to the Brophy mischief. She possesses a beautiful singing voice, a talent she hides because her family holds traditional views about gender roles. She introduces Isaiah to her community after hearing the story of his lost family.

Lucifer is a hairless sphynx cat living with the Brophy family. He hunts the local mice and acts as a constant physical threat during their food scavenging trips. He enjoys toying with his prey, batting smaller animals around rather than killing them quickly.

Dr. Ledbetter is the head scientist at Lamina Labs. He views the animals in his facility merely as test subjects devoid of individual value. He conducts unauthorized, painful experiments on mice, aiming to study their heightened intelligence and unusual genetic traits.

Mr. Brophy is a janitor working at Lamina Labs. His habit of leaving doors open allows Isaiah and his family to initiate their escape. He also owns the suburban house where Mikayla's mischief resides. His generally apathetic nature results in a messy home, providing abundant food scraps for the mice living under his floors.

Abe is one of Isaiah's 96 biological siblings. He has electric crimson fur and a bold, humorous personality. He remains trapped in the laboratory after the initial breakout goes wrong, enduring ongoing experiments by the scientists.

Winnie is Isaiah's sister, characterized by her bright chartreuse fur. She exhibits warmth and kindness toward her family members as they endure captivity in the research facility.

Benji is Isaiah's brother and the de facto leader among the siblings. He possesses a strategic mind and orchestrated the family's first escape attempt from the laboratory using a propped-open door.

James the Wise is an elder in the Brophy mischief. He oversees the community's operations, leads blessings before meals, and offers Isaiah a temporary home after hearing about his plight.

Gabriel is a muscular gray mouse in the Brophy mischief. He gives Isaiah a tour of their burrow and teaches him how to scavenge for food in the human house above. He frequently risks his life during kitchen raids.

Gwindell is Gabriel's sister. She participates in the nightly food raids in the Brophy kitchen, facing the constant danger of traps and predators to feed her family.

Dwayne is the human son of Mr. Brophy. He is a profoundly messy eater who inadvertently sustains the entire mouse colony under his house by dropping food like bacon, cookies, and cream horns on the floor.

Dolce is a small pet dog belonging to Hailey's family. While generally harmless, her loud barks present a noise hazard to the small mice attempting to sneak through the house.

Red Boy is a cat living in Hailey's house. Unlike the aggressive Lucifer next door, Red Boy is lethargic and shows little interest in hunting, making Hailey's room a relatively safe space for mice.

Melissa is a bully at Hailey's middle school. She specifically targets Hailey because of her unusual white hair and pale features, calling her cruel names and causing Hailey significant emotional distress.

Father Ed is the priest at the local church Hailey attends. He presides over a special ceremony intended to bless the pets of the congregation.