What facts are we told directly about the sniper? What can you infer about the character in "The Sniper"?
We are told that the sniper is a Republican, and, in the context of Irish history, this means that he fights for the Irish Republican Army (the IRA), and he believes that all of Ireland (including Northern Ireland) should be politically independent of England. The IRA also embraced violence as a necessary way to accomplish the goal of Irish independence. To say that the sniper is "a fanatic," is not necessarily a good thing; to be a fanatic means that one could also be described as a zealot, someone who fights, perhaps blindly or without considering the consequences of one's zealotry. We are also told that his eyes have a "cold gleam." Again, we might infer that this denotes a kind of emotional detachment from what he's doing. He shoots without emotion, without considering the life of the person who he shoots: all that matters to him is the cause.
We are also told that, after shooting the sniper across the street, "The lust of battle died in him. He became bitten by remorse [...] [and] he revolted from the sight of the shattered mass of his dead enemy." From these words, we can infer that the sniper may choose not to continue fighting in this way. We know that he decides to report to his company commander: perhaps the sniper will tell him of the change in heart he's had. It is ironic, of course, that the sniper rethinks his violence before he learns that it is his own brother he's killed; if only he'd had these thoughts sooner, he might have avoided this tragic confrontation.
Not much direct information is given about the character of the Republican sniper in the short story "The Sniper". The reader is told that the sniper had "the cold gleam of the fanatic". A fanatic is a person who supports a cause with a lot of enthusiasm. A person who will do whatever it takes to support that cause, even if it's illegal or is dangerous to risk life and limb. Therefore, by telling us he has the look of a fanatic, we can infer that he will do what it takes to fight for the cause of his army. The reader can also see that the sniper must be young based on the following quote from the story, "His face was the face of a student, thin and ascetic..."
That's really about it for direct characterization. The rest of the characterization is given to us indirectly based on the sniper's actions, and some physical characteristics. We can infer that he is an experienced soldier. In the story, we are told that his eyes were "deep and thoughtful, the eyes of a man used to looking at death." We can infer that this isn't his first stint in combat. He has fought numerous times prior and has probably killed many times or has seen many killed. Also, he kills the sniper in the turret with one shot and the female informer in one shot. His aim is impeccable. That is from years of experience.