Life of Harold
Harold is a special and important character in the Fallout story. Originally, he himself was a vault dweller. He emerged from Vault 29 in 2090 and began a successful career as a trader and merchant, venturing across the wastes. Eventually though, as he became an important player in the Hub, he began to notice the increasing frequency of mutant animals attacking his caravans. Frustrated, he decided to deal with this. He and a group of other adventurers--among them a man named Richard Grey, a doctor living in the Hub who was equally perplexed by the strange mutants--tracked the mutant population and eventually found its source: an old Military Base that seemed to be spawning mutant critters.
Inside, most of them found death, killed by all the mutants that littered the base as well as its automated security systems. Harold, Richard and a few others made it fairly deep into the base, where they found immense vats filled with a strange thick, green solution that seemed to be mutating the animals. A large robotic arm knocked Grey into a vat, where Harold assumed he died. Harold himself was knocked unconscious. He awoke some time later out in the desert, already starting to mutate. A caravan eventually found him and brought him back to the Hub, where he settled into a destitute existence.
The Vault Dweller first met him in the Oldtown section of the Hub. Oldtown was the part of the Hub where the poor lived as well as a moderate ghoul population. Harold is one such mutant and he makes a living begging for spare change. In exchange for some money, Harold provides the Vault Dweller with a great deal of information, mostly dealing with the Mariposa Military Base and Richard Grey, as well as the deathclaw that lives near the Hub that Butch Harris, leader of the Far Go Traders, needs you to deal with.
Many years later, the Chosen One, grandson of the Vault Dweller, runs into Harold again in Gecko, the ghoul town not far from Vault City. After the destruction of the Hub by the fleeing mutant armies following Fallout, as well as the destruction of the Necropolis, most of the ghoul populations of those two towns migrated far north to form a settlement around an old nuclear power plant built by Poseidon Oil before the War. Anyone else would likely be killed by long term exposure to radiation, but the ghouls just find it pleasant. When Harold arrived, the plant was being run dangerously stupidly. Harold quickly took over from the well-meaning but inept leaders of Gecko and got the reactor into some kind of working order. Sometime between when the Vault Dweller left Harold in Fallout and when the Chosen One meets him in Fallout 2, a small tree (which he calls Herbert) began to grow out of the side of Harold's head, showing that even the sterile mutants can be the source of some kind of life. It is an entirely new species of tree - unique and special, just like Harold. When the Chosen One arrives in Gecko, Harold asks him to help fix the reactor. To do so, the Chosen One must secure the aid of Vault City's officials, which is no mean feat.
What happens to Harold after Fallout 2 ends is unknown, but given the long lifespan of mutants and the fact that Harold is near and dear to the heart of Fallout fans, it wouldn't be surprising if he returned for a third time.
Nature of Harold
To the untrained eye, Harold appears to be a plain old ghoul. This is not so, however. Ghouls are the result of massive radiation damage to a human body: Harold is a product of the Forced Evolutionary Virus. No, he's not a super mutant, but the result of a unique combination of radiation damage from constant low-level exposure, indirect exposure to FEV (actually, we don't know just how Harold was infected with FEV as he blacked out for a while) and a fair amount of random chance. He's not a ghoul, he's certainly not a super mutant. To quote Tim Cain, "Harold is special."
Appearances in games
Harold appeared in Fallout 1, Fallout 2 and a horrible appearence as a hidden playable character in Brotherhood of Steel. He was voiced in the first two games by Charles Adler. He was to appear in Van Buren, the cancelled Fallout 3 project by Black Isle.
Quotes
"How did you survive?" "Didn't. Got killed."
"I'm looking pretty good for being dead."