Tom is the ten-year-old son of the Little family. Standing just a few inches tall with a long, soft tail, he possesses a brave and optimistic personality similar to his father's. He takes an active role in protecting his family, volunteering for dangerous tasks like dressing in a disguise to trick the humans. He approaches problems with an open mind, preferring creative solutions over violence.

William T. Little, known as Mr. Little, is the six-inch-tall patriarch of the tiny household. He leads his family with steadfast optimism and relies on his deep experience to keep them safe from the dangers of the massive human world. He takes his responsibility as a provider seriously and carefully monitors the house for threats, issuing strict safety rules when rodents appear.

Uncle Pete is a seasoned elder of the Little family who walks with a limp following a devastating battle with rodents decades earlier. As a veteran of the Mice Invasion of '35, he maintains a highly cautious and pessimistic view of the world. He prefers to keep his sword sharp and advocates for direct, aggressive action against threats rather than creative problem-solving.

Lucy is the eight-year-old daughter of the Little family. She is highly anxious and openly expresses her intense fear of both felines and rodents, preferring to stay safely hidden in her room when danger arises. Despite her frequent worry, she possesses a strong loyalty to her family that allows her to act bravely when her loved ones face immediate peril.

Granny is the eldest resident of the Little home. She provides essential survival materials by repurposing discarded human items into clothing and disguises for her family. Her age grants her significant wisdom, allowing her to accurately predict the consequences of the new human renters' poor housekeeping long before problems actually arrive.

Mrs. Little is the quiet, protective matriarch of the tiny household. She cares deeply for her children's safety and supports her husband's decisions regarding family security. She relies heavily on the routines established by the human owners of the house, particularly enjoying the meals prepared by the human mother before the new renters arrive.

Mr. Newcomb is a human city-dweller who rents the house for the summer to paint. He strictly refuses to clean up after himself, allowing garbage to pile up and dirty dishes to sit out. His negligence creates a severe safety hazard for the tiny residents hidden in the walls by attracting dangerous rodents.

Mrs. Newcomb is a human writer whose evening story-reading sessions inadvertently provide nightly entertainment for the tiny family listening through the walls. Like her husband, she neglects basic housework until she is personally frightened into demanding action.

George Bigg is the human owner of the property where the tiny family secretly resides. He remains completely unaware of his hidden tenants, mistakenly believing his house simply maintains itself well. He leaves for a three-month vacation, triggering the central conflict of the story.

Mrs. Bigg is the human matriarch of the household. Her regular cooking and knitting habits provide a steady, reliable source of food and craft supplies for the hidden family living in her walls.

Henry Bigg is a member of the human family that owns the house. His discarded belongings, such as damaged clothing, provide vital raw materials for the tiny residents to survive.

The Cat is a feline brought into the house by the summer renters. As a large predator, she represents a massive physical threat to the tiny people living in the walls. Her predatory instincts make her an immediate danger until she is approached in a calm, non-threatening manner.