I don't know as I haven't read the story, so I can't say for sure what the main characters are.
I have no knowledge of the 'the tragedy of birlstone full story', but typically, main characters in a tragedy might include those directly affected by the tragic events. It could be a local hero who tries to save the day but fails, or an ordinary citizen whose life is turned upside down by the events in Birlstone. There could also be authority figures like the police or the mayor who play important roles in the story.
Since I'm not acquainted with 'the tragedy of birlstone full story', it's difficult to name the main characters. But usually in a story, main characters could be the ones who drive the plot forward. They could be people like the victims, the villains if there are any, or the ones who are trying to solve the mystery or deal with the tragic situation.