The Spelling Bee
The Bee is one of the first characters to tell Milo about the importance of education. Since this is one of the main takeaways of The Phantom Tollbooth, we think he's worth talking about.
The Spelling Bee exactly what he sounds like: a huge bee that's obsessed with spelling words. He can't go a full sentence without spelling something. And while it sounds like he has had an interesting life, we only get to learn a little bit of it. "You see," he tells Milo, "years ago I was just an ordinary bee minding my own business […] Then one day I realized that I'd never amount to anything without an education" (4.38).
If you think about it, the Spelling Bee is one of the few characters who's really explicit with Milo about the value of learning. And because he says this at the beginning of the book, we're probably more in tune to all the learning Milo does throughout the rest of the story.