Ralph is a unique mouse who lives at the Mountain View Inn and rides a tiny red toy motorcycle. He possesses the rare ability to speak to lonely children and power his vehicle by making engine noises. Considering himself more civilized than his unruly outdoor relatives, Ralph prefers the safety of his grandfather clock nest but takes risks to protect the humans who treat him well.

Ryan is the lonely son of the Mountain View Inn's housekeeper. Being new to the town of Cucaracha and living in an isolated ski resort, he lacks social connections and uses his secret friendship with Ralph to cope. He eagerly smuggles the mouse into school, hoping to impress his classmates and establish a place for himself in the social hierarchy of Room 5.

Brad is Ryan's classmate at J. Sneed Elementary. He rides dirt bikes and projects a smug, indifferent attitude that hides his private family struggles. He actively doubts Ryan's claims about Ralph's intelligence, preferring to design difficult traps rather than fair tests for the classroom's unusual guest.

Matt works as the loyal custodian of the Mountain View Inn. He is the only adult capable of communicating directly with Ralph, maintaining a gentle empathy that allows him to understand the animals living in the building. He actively covers for the mice, even when his kindness puts his own employment at risk.

Miss K is an energetic educator at J. Sneed Elementary School. She excels at turning unexpected classroom disruptions into comprehensive learning opportunities, immediately adjusting her lesson plans to study mice when Ralph appears. She prioritizes teaching empathy and critical thinking alongside traditional academic subjects.

Mr. Minch is the strict manager of the Mountain View Inn. He shows little patience for the aging hotel's maintenance issues or its employees, enforcing rigid rules about cleanliness that threaten the resident mouse population.

Mrs. Bramble is Ryan's mother and the primary housekeeper at the Mountain View Inn. She works hard to maintain her job and provide for her son, remaining completely unaware of the talking mouse living under the lobby clock.

Melissa is a student in Miss K's class at J. Sneed Elementary. She inadvertently aids Ralph by leaving her winter boots at school, providing him a dark place to sleep, and later writes a creative story about him getting stuck in the laundry.

Gordon is a student in Miss K's class who approaches the mouse project from a strictly textbook perspective. He initially frightens Ralph with statistics about extermination but later demonstrates fairness by defending the mouse's unconventional method of solving the classroom maze.

The Reporter works for the Cucaracha Voice newspaper. He spends only a few minutes observing Miss K's classroom before leaving to write a sensationalized, inaccurate article claiming the school suffers from a massive rodent infestation.

The Principal of J. Sneed Elementary handles administrative duties and responds to community concerns. Following the exaggerated newspaper article, he takes a measured approach to pest control, ordering only five traps instead of a massive extermination.

Brad's Father works as a tow truck driver and raises his son alone following a divorce. He provides a simple, quiet life for Brad, though his busy work schedule contributes to his son's underlying feelings of isolation.