The main character is a boy. And of course, Shiloh, the dog, is also a main character. There are also the boy's family members like his parents who play important roles in the story.
The main characters likely include Shiloh, who is probably the central figure. Then there might be his parents or guardians. Their roles would be important as they interact with Shiloh and influence the story's plot. Maybe there's also a neighbor or a schoolmate who plays a significant part in the events that unfold in the 'Shiloh short story'.
The drummer boy is of course the most important character. He is the central figure around which the story revolves. The soldiers in his unit are also important characters as they interact with him, and their relationship with the drummer boy shapes the story. And the officers who lead the unit are important too as they give commands and are part of the military hierarchy that the drummer boy is a part of.
Shiloh is most definitely fiction. It has elements of creativity and imagination that aren't based on real events or people. It's a creation of the author's imagination.
The significance could be that Shiloh was a major battle in the Civil War. Their meeting there might symbolize the convergence of different fates or storylines against the backdrop of this important historical event. It could also be a place where key decisions are made or relationships are formed that will drive the rest of the plot in the novel.