Evan is a kind, cautious fifth grader living in Martinville. He relies heavily on daily routines, like stopping by a specific tree on his route to school, to manage his anxiety about growing up and transitioning to middle school. When he borrows two books from the little free library and realizes they survived a massive fire twenty years prior, his natural curiosity drives him to investigate the mystery.

Mortimer is a large, six-toed orange cat who previously lived in the Martinville Library before it burned down twenty years ago. He takes his self-assigned duty of guarding the newly built little free library very seriously, even refusing to go indoors at night so he can sleep near the books. Gentle by nature, he refuses to hunt mice and often longs for his lost sister.

Al is the former assistant librarian of the Martinville Library, a title she cherishes deeply. She currently lives in the local History House, where she cooks and cares for the ghosts of a former library director and a patron. Lacking her original professional purpose, she decides to construct a little free library out of an old cheese cupboard to once again share books with the town.

Evan's father works as an exterminator in Martinville, though he secretly releases the mice he is hired to trap into the mountains instead of killing them. He is a mild-mannered, quiet man who avoids conflict and frequently deflects questions about the past. Having worked as an intern at the Martinville Library during his youth, he holds a deep love for books but acts highly suspicious regarding the fire that destroyed the building.

Rafe is Evan's best friend and fellow fifth grader. He is generally more relaxed and forward than Evan, pushing him to investigate the strange circumstances surrounding the little free library. Rafe holds a strong interest in gardening and uses the library to borrow books on growing tomatoes.

Ms. Scoggin is the ghost of the former Martinville Library director who died in the fire two decades ago. She continues to reside in the History House, maintaining her strict, gruff demeanor and relying heavily on a daily schedule. She frequently issues orders as though she is still running the library.

Mr. Brock is a ghost who perished in the Martinville Library fire. A former library patron, he now spends his afterlife in the History House, passing the time by reading and critiquing various books.

Petunia is an orange cat and Mortimer's sister. She lived with him in the Martinville Library before the devastating fire. Her current whereabouts are unknown, leaving Mortimer to worry about her fate.

Mr. O'Neal is Evan's fifth-grade teacher. He actively supports his students' intellectual growth, noticing Evan's interest in mystery books and encouraging him to follow his curiosity. He holds his own memories of the town before the library was destroyed.

Demetri is an older student who attends the middle school that Evan is nervous about entering. He is friendly and reassuring, offering Evan leftover graduation brownies and promising to sit with him on the bus next year.

Fred is a local mouse who visits the little free library. He complains about the difficulties of foraging for food in the wild and shares gossip about other animals in neighboring towns.

Anne is a resident of Martinville who discovers the little free library. She brings a suitcase full of books to donate and takes the time to read poetry aloud, bringing comfort to those listening.