Since I don't know the details of the story, I can't say for sure. But it might be the diaper - wearing boy himself as a main character. There could also be a camp counselor or a fellow camper who plays an important role.
I'm not certain, but perhaps the main character is the boy in the diaper. And there could be other characters like his parents who sent him to the camp, or some kids at the camp who either befriend him or bully him because of his diaper - wearing situation. There may also be a camp leader who has a significant influence on the boy's experience at the camp.
Without knowing the story well, the obvious main character would be the 'diaper boy'. He is likely at the center of whatever events unfold at the camp. Additionally, there could be a group of friends he makes at the camp who are also main characters in terms of how they interact with him. And a strict or kind camp director could also be a main character, especially if they are involved in decisions regarding the boy's stay at the camp.
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