Milo is a young boy who suffers from chronic boredom and sees no value in education or daily life. His perspective shifts dramatically when he receives a magical turnpike tollbooth, complete with a map and currency. Driving his small electric toy car through the tollway, he enters a vibrant, bizarre territory where his mind is constantly challenged. As he travels, he receives the grand quest of restoring order to the divided Kingdom of Wisdom.

Tock is a dedicated watchdog whose body literally consists of a giant, ticking alarm clock. He patrols the featureless Doldrums to ensure no one engages in chronic laziness, frequently sounding his alarm when time is being squandered. He brings a strong sense of rationality to Milo's strange journey and acts as the boy's faithful, protective friend.

The Humbug is a large, well-dressed beetle who prides himself on his supposed aristocratic lineage and invents heroic stories about his past. Despite presenting himself as an important friend of the king, he is actually a sycophantic blowhard who constantly attempts to agree with whoever is speaking. Through a misunderstanding, he accidentally becomes the appointed guide for Milo's perilous quest.

King Azaz the Unabridged is the gruff and impatient ruler of Dictionopolis, the city where letters grow on trees. He is a very large, stout man with a long gray beard, wearing a small crown and a robe heavily embroidered with the alphabet. He maintains a fierce, stubborn conviction that language is superior to mathematics, fueling a bitter feud with his brother.

The Mathemagician is the founder and ruler of Digitopolis and the estranged brother of King Azaz. Clad in a pointed cap and a robe covered entirely in equations, he proudly oversees the mining, polishing, and exporting of numbers. He uses a giant eraser-topped pencil as a magical staff and firmly believes that logic and arithmetic surpass words in value.

Rhyme and Reason are two wise, beautiful, and gentle princesses who were adopted by the old prince of the Kingdom of Wisdom. Before their banishment, they were renowned across the land for their ability to settle all controversies fairly and harmoniously. Their absence causes a chaotic rift between the advocates of logic and creativity, prompting Milo's grand rescue mission.

Alec is a young boy who floats effortlessly above the ground because his family members are born with their heads at their adult height, growing downward as they age. This unique physiology gives him a constant, elevated perspective on the world. He has the ability to see through solid objects but ironically struggles to notice obstacles directly in front of his nose.

The Duke of Definition, Minister of Meaning, Earl of Essence, Count of Connotation, and Undersecretary of Understanding are tall, lean aides dressed in elegant silks and top hats. They speak in a continuous chorus, each offering a different synonym for the same idea without actually advancing the conversation. They oversee the word trade for the king but care little for how language is practically used.

The Whether Man is a cheerful but entirely useless functionary who greets travelers at the roadside junction of Expectations. A small, bearded man in a long coat, he completely refuses to make up his mind about anything. He prefers to ponder "whether" there will be weather rather than attempting to predict what type it will be.

The Lethargarians are tiny, humanoid creatures who inhabit the featureless Doldrums. They constantly camouflage themselves to match their background and spend their entire lives yawning, napping, and avoiding any form of physical effort. They strictly enforce local ordinances against thinking and laughing.

The Spelling Bee is a giant, highly educated insect capable of spelling almost any word in existence. He takes immense pride in his academic abilities and insists that proper spelling is a crucial skill. His zealous dedication to his singular talent makes him somewhat annoying and puts him directly at odds with less genuine characters.

The Dodecahedron is a geometrical being with a 12-sided head, featuring a different emotional expression on each specific face. As an enthusiastic resident of Digitopolis, he is incredibly precise and deeply passionate about mathematics. He finds the human world of singular faces and non-mathematical logic to be hopelessly confusing.

Faintly Macabre, the not-so-wicked Official Which, is an elderly lady and the great-aunt of King Azaz. She once held the prestigious role of selecting which words the populace should use, but her growing miserliness eventually led her to decree absolute silence upon the city. Now deeply remorseful, she languishes in prison, waiting for the kingdom to regain its sanity.

Dr. Kakofonous A. Dischord is a doctor of dissonance with huge ears who wears a traditional lab coat and a head mirror. He specializes entirely in creating terrible noises, concocting sound-based medicines to cure people of their desire for beautiful auditory experiences. He thrives on the mechanical clamor of modern society and operates on the outskirts of the Valley of Sound.

The Soundkeeper is the wise and generous ruler of the Valley of Sound. Covered in thousands of tiny, chiming bells, she oversees the creation, archiving, and distribution of all auditory experiences in her fortress. Deeply disappointed by her people's growing disregard for beautiful sounds, she has enacted a strict edict banning all noise.

Chroma the Great is a tall, gaunt, and highly elegant conductor who commands the enormous orchestra responsible for bringing visual color to the world. Through his precise musical direction, he paints the environment with the shades of dawn, day, and night. He is intensely dedicated to his craft but chronically exhausted by the endless daily schedule.

The Demons are a terrifying assortment of monsters that inhabit the dark wilderness of Ignorance. They include the faceless Terrible Trivium, the camouflaged Gelatinous Giant, and the jittery Demon of Insincerity. Each creature embodies a specific mental trap or pitfall of learning designed to derail travelers from achieving their goals.

The .58 Boy is exactly a fraction of a child. He explains that he belongs to a statistically average family consisting of precisely 2.58 children, naturally representing the fractional part. He happily embraces his mathematical existence, firmly believing that even impossible averages hold logical value in the grand scheme of numbers.

The "awful Dynne" is Dr. Dischord's boisterous, cloud-like assistant who was raised from an orphan living in an empty soda bottle. He roams the valley with large sacks, excitedly collecting terrible noises for the doctor's cacophonous medicines. He easily sulks when people fail to appreciate truly awful sounds.

Officer Shrift is a heavily built police officer in Dictionopolis who is roughly two feet tall and twice as wide. He operates as a highly efficient but entirely illogical one-man justice system, acting sequentially as judge, jury, and jailer. He delights in pronouncing people guilty and handing out absurdly long prison sentences.

Canby is a magical inhabitant of the Island of Conclusions who alters his physical state to literally be "as [adjective] as can be." He melancholically explains the rigid rules of the island to travelers who have unthinkingly jumped there, noting that the only possible method of escape is a painfully long swim.