The main character, Milo is a little boy who goes through all of his days in a state of horrible boredom. This routine changes when Milo takes a trip through the mysterious make-believe tollbooth that appears in his bedroom one day.
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Milo's friend Tock is a literally a "watchdog." A giant clock makes up part of his body, and he constantly makes ticking noises. He patrols the Doldrums and stops people from wasting time.
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The Humbug is an insect who lives only to flatter people—especially himself. The Humbug is ignorant about everything from math to geography and proves himself the fool by his constant attempts to say intelligent things. After trying to brown-nose his way to favor with King Azaz, he accompanies Milo and Tock on their journey.
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King Azaz is ruler of the realm of letters and words. Azaz and his brother argue over which is more important—numbers or letters, and they banish the princesses Rhyme and Reason. Once he realizes the foolishness of his squabble, King Azaz sends Milo to rescue the princesses.
Azaz's brother, the Mathemagician, lives in a world of numbers. Unlike Azaz, the Mathemagician has doubts about releasing Rhyme and Reason.
The two princesses were adopted by the King of Wisdom and raised alongside Azaz and the Mathemagician. When Azaz and the Mathemagician asked them to determine whether numbers or letters are more important, Rhyme and Reason say each is equally valuable. The brothers then imprisoned the two princesses in the Castle in the Air.
The Which, Faintly Macabre, has been imprisoned since Rhyme and Reason disappeared. It was once her duty to select the words to use for every occasion, but she became corrupted by her power and began to horde the words for herself. Faintly tells Milo the story of the imprisoned princesses and inspires him to broach the subject with King Azaz.
Milo first meets Alec Bings in the Forest of Sight, where Milo sees a boy floating several feet off the ground. Alec explains that in his family, everyone's head remains at the same height from the day they are born until the day they die and that their legs grow toward the ground. Alec has the special ability to "see through things" and can see anything except that which is right before his eyes.
Conductor of the great color orchestra in the Forest of Sight, Chroma makes sure all the colors of the day are properly handled. When he decides to take a rest, Milo makes a mess of the colors of the day.
Dr. Dischord, a quack doctor, prescribes medicines of terrible noises to all of his patients and has an assistant, a smoke monster named Dynne. Dischord and Dynne invent new sounds, peddle noise pulls, racket lotions, clamor salves and hubbub tonics in the Valley of Sound.
Once ruler of the Valley of Sound, the Soundkeeper becomes dismayed with the lack of appreciation of beautiful sounds and the rise of Dr. Dischord's terrible practice. In protest, she cuts off sound and retreats to the fortress where she keeps all sounds made since the beginning of time.
The Dodecahedron has twelve different faces wearing twelve different emotions. He leads Milo and his companions through the numbers mine, where workers chisel out gemlike digits, to the city of Digitopolis.
More nuisance than demon, the Everpresent Wordsnatcher is a bird who flutters around the Mountains of Ignorance turning the words of others around to illustrate his own cleverness.
The Terrible Trivium is a demon with no facial features. He lives in the Mountains of Ignorance and preys upon travellers, convincing them to undertake tasks that can never be completed.
The Demon of Insincerity looks like a cross between a beaver and a kangaroo. He tries to scare Milo and his companions off their path through the Mountains of Ignorance by throwing half-truths at them, which are only dispelled when they see this demon for what he really is.
The Gelatinous Giant is so huge that Milo first mistakes him for a mountain. He is the epitome of spinelessness. He hides in the Mountains of Ignorance and tries to look exactly like everything around him because he thinks it is "unsafe" to be different.
The Senses Taker spends his days in the Castle in the Air trying to rob people of their senses by bombarding them with detailed questions. His appearance as an ink-stained old man perched over an enormous book deceives Milo into thinking his purpose is anything other than wasting time.
Officer Shrift is twice as wide as he is tall. In Dictionopolis, he works as a police officer, judge, and jailer all at the same time. Officer Shrift has a habit of sentencing people to millions of years in prison then immediately forgetting about them.
A peculiar fellow who says everything three times, the Whether Man is the caretaker of Expectations. He is so busy thinking about what could be and why that he never seems to go anywhere or get anything done.
The Lethargians, minute creatures, live in perpetual boredom in the Doldrums. They change colors to match their surroundings and sometimes enforce laws against thinking and laughing.
Though he is a giant bee, the Spelling Bee is a self-taught master of spelling and enjoys randomly spelling the words he hears or speaks.
The result of a statistic, the Half Boy is really the leftover .58 from the 2.58 children the average family has. He believes in the reality of averages and likes to spend his time on the staircase to Infinity.