Henry is a pragmatic, independent third-grader living on Klickitat Street. He generally feels his life is ordinary and boring, but this changes abruptly when he decides to adopt a stray dog. Though he approaches problems with determination, his literal-minded thinking sometimes creates unexpected complications when dealing with the expectations of adults.

Ribsy is a scrawny, stray mixed-breed dog who gains his name from his gaunt appearance. After winning Henry's affection with a single bite of ice cream, he becomes a permanent fixture in the boy's life. He behaves like a typical canine, bringing both companionship and a steady stream of mischief to Klickitat Street.

Henry's parents maintain a nurturing and secure household. While Mrs. Huggins handles the day-to-day domestic responsibilities and worries about practical matters like fleas, Mr. Huggins steps in to offer firm advice during moments of crisis. Together, they support their son's growth by letting him face the consequences of his choices.

Scooter is a fifth-grader who lives in Henry's neighborhood. As an older child, he frequently tries to assert his superiority by showing off his possessions or boasting about his abilities. His need to impress others masks a deeper insecurity, often making him a source of friction for the younger children.

Beatrice "Beezus" Quimby is a sensible girl who lives on Klickitat Street with her family. She spends much of her free time playing outside with her best friend, establishing a distinct social circle separate from the boys in the neighborhood. She stands her ground when the boys attempt to mock or disrupt her games.

Ramona is Beezus's highly imaginative younger sister. She is an energetic presence on Klickitat Street, often inserting herself into the older children's games and pretending to be different animals.

Mary Jane is a neighborhood girl who takes great pride in her appearance and school activities. She enjoys practicing acrobatics and tightrope walking with Beezus, forming a united front when the neighborhood boys attempt to ruin their fun.

Robert is one of Henry's neighborhood friends and classmates. He enjoys attempting physical stunts and shares Henry's tendency to wander the neighborhood looking for distractions.

Mr. Pennycuff owns the local pet shop and serves as a knowledgeable resource for Henry. He encourages the boy's interest in animals and offers fair deals that help Henry manage his growing collection of pets.

Mr. Grumbie is an adult neighbor who enjoys fishing trips. His need for live bait provides a timely economic opportunity for the children in the neighborhood.

Miss Roop is a schoolteacher tasked with directing the holiday stage production. She holds traditional expectations for her students and expects full participation in school events, regardless of their personal comfort levels.