The reader is told that the sniper is a young Republican who has the face of a student and a thin body. His eyes are cold, deep, and thoughtful. The sniper also smokes and drinks whiskey while he is on top of the roof. The sniper is also an excellent shot. He successfully kills three people in three shots throughout the short story. The sniper is also intelligent and savvy. He tricks the enemy sniper into thinking he is dead, which gives him an opportunity to kill the opposing sniper.

Following his initial excitement after killing an enemy sharpshooter, the sniper becomes remorseful and begins cursing the war, himself, and everybody. He evens throws his revolver in disgust. This indicates that the young Republican sniper is sick of fighting and regrets participating in the civil war.

The sniper makes the bold decision to enter the street in order to identify the enemy sniper he has killed. This indicates that the young Republican sniper is quite reckless and curious. Unfortunately, the sniper discovers that he killed his brother. This significant piece of information informs the reader that the sniper's family was more than likely politically split between the Republicans and Free Staters. It is at this point in the story that the internal conflict is revealed. The sniper struggles with the emotional and mental toll of participating in a civil war against a family member. He also struggles with the guilt of killing his own brother in battle.

What facts are told directly about the sniper? And what can you infer about his character?

We are told a number of things about the sniper, and from those and his actions we can infer more about his character.

We are told what his face looks like ("thin and ascetic"), and his that his eyes gleam coldly, like a fanatics. When he gets excited, he does not eat. We are told he smokes, that he's injured in the course of the story, and that he is frightened, then that the fear goes away. He's an Irish Republican.

From these facts and the action, we can infer the following:
He's completely dedicated to his cause, so much so that he'll kill without confirming the identity of those he's killing.
He's reflective about the risks he faces, but not fully wise. (He does smoke, despite the risk of exposure.)
He's a good shot.
He has a high pain threshold.
Either his family is split on Irish political issues, or either he or his brother misidentified their targets.