We first meet Charles Halloway, Will's father, in Chapter 2. He is described first by Will as "not grandfather, not far-wandering, ancient uncle, as some might think, but…my father" (2.21). (At 54, we would hardly call Charles elderly, but we suppose that, to thirteen-year-old Will, it seems ancient.) And the fact is, sure, Charles is past his prime and he feels it. His longing for the youth and boyhood that Will and Jim enjoy is palpable in the first part of the novel. He always wants to run with Will and Jim, to join them in following a wind "to all the secret places that were never so secret again in life" (3.1). It's important that Charles feels this longing, because it means that he, too, is sorely tempted by the carnival. A quick ride on the carousel, after all, should solve all his problems. There is a significant struggle here, an internal one, really, in Charles's refusal to give in to his most intense desire.
Charles works as the janitor at the town library, where he can exercise his love for books. The library becomes a refuge for Charles and the two boys once the evil of the carnival has been exposed and must be combated. In the library, Charles holds forth on the secrets of goodness and evil, love and life. These passages (Chapters 37 to 40) are some of the most important in the novel for they help frame the book's central issues of goodness and evil. It's particularly important that Charles is the one to expound on these issues to the boys – in this way, he acts as a central guide for the two of them (and for the reader, who is also learning what's up with the carnival). It's also worth noting that Charles is able to grasp these concepts, and to explain them to Jim and Will, precisely because he is old. Though youth seems desirable, there's something to be said for the wisdom of later years.
What Charles's character is ultimately able to demonstrate is that youth can be a state of mind, not just a physical thing. Charles's assumption of leadership in the battle against the carnival successfully transforms him from the remote, middle-aged man we first meet to a jovial, heroic youthful man who has no trouble keeping up a footrace with two young boys. As such, Charles is by far the character who alters the most over the course of the novel – not physically like Miss Foley and Tom Fury, but emotionally. He ends the novel by showing us that fifty is the new thirty. Or thirteen.