Protagonist and narrator of The World According to Humphrey, Humphrey is a golden hamster, purchased as a class pet from Pet-O-Rama by Ms. Mac. Humphrey is given a tiny pencil and notebook by Ms. Mac, and he learns to read and write in a matter of weeks. Though he understands and writes in English, his verbal communications come across as nothing but squeaks to his human companions. Humphrey’s mission in life is to help the students of Room 26 to learn about themselves. He takes this job seriously, coming up with plans whenever he detects a problem. Humphrey idolizes Ms. Mac, but fears Mrs. Brisbane, who dislikes him. Humphrey goes to a different student’s house every weekend.

Throughout the novel, Humphrey helps many people. Sayeh Nasiri, an immigrant girl who never speaks up in class, finds her voice once Humphrey catches her parent’s attention and spurs them to practice English. Humphrey gets A.J.’s family to watch less television and spend more time together, and he helps rekindle A.J. and Garth’s friendship. He helps adults as well. He gets Principal Morales’s children to work together, causing them to respect their father more. Humphrey helps Aldo, the night janitor whom he fosters a friendship with, to find a girlfriend whom he later marries.