This unnamed assailant is a reccuring character throughout the series. He began his tale in "Wasteland Stories" (Episode 2) by taking a sniper rifle from his first victim. He later managed murder 3 resting rival mercenaries.
The Lone Sniper is most likely amother bandit. However, it is possible he is a mercenary soldier himself. He has no official affiliations, but he could be working for The New Order or even New Russia
In the episode "New Russia", he is seen to be stalking a Commonwealth outpost. When the New Russian troops launched an offensive in the area, he began taking out numerous Commonwealth soldiers. He did not reveal himself, as he feared the New Russians would kill him as well.