The sniper in the story is mysterious because his only identity is that he is a soldier. The author takes us through the action of the story to express his feelings about war, especially civil war. A civil war is one in which your enemy is a person of your own country. In the American Civil War, Americans fought Americans, sometimes even their brothers. This is true of the civil war in Ireland.
The most dramatic part of this story is the surprise ending where the sniper discovers that the other sniper he has just killed is his brother. He thought maybe he might know him and that's why he's curious to see who he is. The reader doesn't know how the sniper feels about killing his brother, underscoring the author's theme of what war does to a person.