Nac: Just what the hell are you doing, anyways?!
Old man: A sword of a different color... harharhar... henhhenhhenh
― Nac, inquiring about the noisy tinkering[src]
The unnamed old man was a seemingly crazy old man that saved the Resistance member Nac from both the Machines and from the serious injuries he sustained. He was often seen tinkering with the machinery he salvaged and had a penchant to collect books about medieval knights and warriors. Even after surviving alone in the Real World, crafting a safe house, and various tools, weapons and armor, the old man seemed to have, as Nac stated, "no comprehension as to his own origins" and "no understanding of the real world".
When Nac suffered some serious injuries after his hovercraft crashed on a dump site near a fetus field, he passed out and was dangerously close to being found in the debris by sentinels. Fortunately, the old man was able to reach him via a tunnel and, so, rescued and dragged him into his underground hideout which was reachable via a small lift.
When Nac next woke up, he found himself in the old man's home facing a deactivated and gutted sentinel. The old man begun to talk to Nac and called Nac a fellow brother of the order, deludedly believing himself and Nac to be some sort of knights, and referring to the plugs in their body, created from when they were both still in their pods, as "silvery bites" suffered from "dragon stings". He also believed it his own mission to destroy these dragons.
Despite his delusions though, he was able to keep Nac alive and helped Nac through his fever by giving him a bed to sleep on, boiled water to drink, and, for food, what Nac believed was cockroach paste. The old man himself mostly ate small animals such as rats. After one of his scavenging trips, the old man even brought back equipment from Nac's hovercraft, including a broken radio which Nac begun repairing.
After a while, the old man begun swinging his sword through their hideout fighting imaginary battles and, after observing the old man's fantasies, Nac eventually tried to broach the truth of their real-world situation to the old man. The old man, however, simply rejected all of it and instead showed Nac proof of his own version of reality. Using a salvaged periscope now attached to the safe house, the old man showed Nac the "dragons" he aimed to defeat which, Nac realized, were actually harvesters.
The old man, committed to his crusade, continued to tinker with the hollowed-out sentinel, wielding a torch he called "a sword of a different color", and welding shrapnel to it as well as putting in large quantities of cyclonite. And then, one morning, he was just gone, leaving Nac a package and taking the sentinel with him. The bundle contained a book called "The Trojan War" as well as a drawing of himself and Nac, their arms over each other's shoulder, while raising their swords, and with the title "FREEDOM" written above the drawing. As Nac suspected, he spotted the sentinel shell in the field attracting a number of real sentinels, and then, shortly, it detonated, causing a huge destruction to the field.
After retelling the story back in Zion, Nac started to believe that the old man wasn't just simply crazy.