The Phantom Tollbooth
Usually when we describe someone as two-faced, we're judging that person for being hypocritical (meaning, they say one thing and do another). Well, the Dodecahedron has twelve faces, so we'd better watch out, right? Not exactly.
Our geometrical friend, the Dodecahedron, changes faces to show off his different emotions. It's as though he has twelve set expressions and just has to rotate them, depending on how he's feeling. Sounds pretty awesome, right? But think about it. If the Dodecahedron has twelve faces, we humans must have, well, thousands! We dare you to count how many you can make.
The Dodecahedron helps introduce Milo, Tock, and the Humbug into the wide world of numbers that is Digitopolis. He serves as their guide into that realm in the same way that Alec leads them through the Forest of Sight. The Dodecahedron explains where numbers come from (not where you might think, it turns out) and introduces the travelers to the Mathemagician. While he's at it, he also helps Milo think about the importance of naming things, and about recognizing people and objects for what they actually are.
Without people (or shapes, as the case may be!) like the Dodecahedron, Milo may have never been able to combat his boredom and become a lover of learning and imagination.