Isaiah is the protagonist and first-person narrator of Word of Mouse. He is a mouse who, along with his 96 siblings, is subjected to experimentation at Lamina Labs, resulting in him having bright blue fur, an ear tag for identification, and exceptional intelligence and abilities, including the ability to speak and type. His journey is about learning to have confidence in himself and his abilities.

At the beginning of the book, he admits that he is the most cowardly of the siblings and easily frightened. After the rest of his siblings are captured and returned to the lab, he is forced to rely on himself more. He ends up in a nearby suburban neighborhood and uses his skills to protect himself. As part of living with the local mischief under the Brophy family’s house, he shows his bravery when participating in nightly raids to acquire food. He also shows his ingenuity and resourcefulness when coming up with a method to carry more food using a napkin, rescuing Gwindell from a mousetrap, and saving them from Lucifer when the cat traps them in the bathroom. Each achievement bolsters his self-confidence and builds his connections to his new friends.