Charles Halloway provides a little lecture series for the boys on the nature of good and evil in the build-up to the final confrontation with the carnival. The lecture is helpful: "Dad, don't stop, thought Will. When you talk, it's swell in here. You'll save us. Go on" (39.20). By virtue of the wisdom he has and his status as a grown-up, Charles serves the boys as a mentor. Remember that one of Will's biggest concerns was that their story would be met with disbelief from the adult community. Charles's statement "I believe" affirms the boys' experience.
We'll throw a curveball at you here in the end of the analysis, however, and point out that Charles is an unconventional mentor given his desire to become one of the boys. This throws into question his ability to really maintain proper distance from the issues at hand.