Obviously, the main character is the boy who is getting fat.
The boy is the main character. In this story, his experiences related to getting fat are the focus. His emotions, his attempts to deal with the situation, and how his life changes as a result of the weight gain are all part of what makes him the main character in this 'boy getting fat story'.
As we don't know the exact 'fat boy expansion story', it's hard to determine the main character precisely. But if we go by the name, the main character is likely to be a boy who is referred to as 'fat boy' in the story.
Well, perhaps the 'boy getting fat story' is a cautionary tale. It might detail how the boy's lifestyle, such as sitting around all day playing video games and constantly snacking, led to his weight gain. It could also explore the consequences he faces, like being bullied at school or having health problems.
Since I don't know the specific 'fat boy chronicles true story', I can't say for sure who the main character is. It could be the person who is sharing their own experiences as a 'fat boy' in the story.
The fat boy and the giant boy are surely the main characters. However, there could be other supporting characters that we don't know about yet. Maybe there are friends or foes of the fat boy and the giant boy that play important roles in the story as well.
Since I don't really know the details of this story, I can't say exactly who Fat Boy is. He could be a friend of Billy Stewart, or a character created by Stewart for a specific purpose in the story.
As we don't have details about this 'fat boy story', we can only assume that the main character is the fat boy. However, there could be other important characters too, like his parents who might be helping him or his friends who could be influencing his life in different ways. But the fat boy is likely to be the central figure around which the story revolves.
If the 'fat man story' is about the Fat Man atomic bomb, then there isn't really a traditional 'main character' in the sense of a person. But if we consider the scientists and military personnel involved in its creation and deployment as key elements, then people like the physicists who worked on the Manhattan Project could be considered important. For example, J. Robert Oppenheimer was a significant figure in the development of the atomic bombs, including the one known as Fat Man.
Since we don't know the specific 'the fat girl story', it's likely the main character is a girl who is overweight.
As I haven't read the story, I can't say for sure. But likely there's the fat boy himself. There might also be his family members like his parents or siblings, and perhaps some friends or classmates.
Obviously, the main character is the old fat mouse.