Thero

In an similar reality, this 13 year old boy snuck into the Vietnam war and fought during its final days in the 1990's where he stayed for 90 days before he lost his left arm to artillary fire. When he was taken back to USA because of his injuries. When he was 17 he was in a military camp training to use his firearm when an portal opened and a dramatic fight came from it, a man wielding fire and another with portals, at one point the man with fire was about to finish the man with portals off when Thero knocked the man with fire to the side. The blast missed the portal man but hit Thero in the left eye at an angle that made it possible for Thero to survive. Because Thero distracted the fire man the portal man was able to "end" the fire man and save the earth. As payment for saving his life, the portal man took parts from his home dimension to fix Thero's eye and even gave him a robotic arm. Both the arm and eye were connected to his brain so that he could control them. As soon as the government found out about the dimensions, they set up an elite company made for the shadows to protect the earth from other dimensional threats, and Thero was one of the first of and best in it.