Answer:
The primary character in The Phantom Tollbooth, Milo, is meant to represent the typical bored child. Milo has everything a child should want in terms of toys and entertainment, yet he is horribly, unchangably bored. He finds everything in his life to be completely uninteresting and has a special disdain for his schoolwork, since he thinks all of it is useless. Milo does not believe that anything he learns—numbers, words, or anything else—is applicable to everyday life.
Milo finds himself under the wing of many characters who impart their different lessons—sometimes intentionally and sometimes unwittingly. Milo is open and attentive to these lessons, largely because he must learn them in order to escape a situation. We can say that Milo is a well developed character.