“Come around to where I can see you (coughs and splutters).”
Harold (who goes by many names: The Lord, Him, The One Who Grows, Gives, and Guides, and The Talking Tree)
 is an FEV mutant - originally a human from Vault 29. He was born in 2072, five years before the Great War. During an exploration of Mariposa Military Base, he was exposed to FEV. Initially, he mutated into what appears to be an ordinary ghoul, but was later distinguished by a tree growing out of his head some years later.
The following is based on Van Buren and has not been confirmed by canon sources.
Harold's home, Vault 29, was initially populated by young children which were brought up by Diana, a human brain connected to a supercomputer, who posed as a goddess to the vault's inhabitants. In cases of population unrest, a test subject would be chosen to be released from the vault early. The subject would return and tell the vault dwellers the condition of the world outside, and whether or not it was safe to leave the vault. Of course, the test subjects would never return, and this fact would deter everyone from wanting to leave again for a short time.
Harold was one such subject, selected in 2090. He was released from the vault, stunned by the security system and picked up by a robot to be taken to the Nursery, where Diana resided. At the Nursery he was given the choice of staying with Diana or exploring the outside world. However, if he chose to explore, she would have to 'condition' him, so that he could not reveal anything about her, or the Nursery to the world.
Harold agreed to stay with Diana, but managed to escape. During his travels he kept the Nursery a secret because he felt it was his duty to the world to keep such a future 'gift' intact until the world was ready for it.
After leaving his home vault when it opened and its inhabitants made their way out into the wasteland, Harold began a successful career as a merchant, venturing across the wastes. Eventually 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 by leading an expedition to Mariposa.
In the year 2102 when Harold was 30 years old, he and a group of other adventurers— among them a man named Richard Grey, who was 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 creatures.
Inside, most of them were killed by the mutants that littered the base as well as by 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 (FEV) that seemed to be mutating the animals. A large robotic arm knocked Grey into a vat, where Harold assumed he died (when actually, he became the Master). 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.
He is now very old, sick, and he tends to ramble on a lot. Some of his information may be out of date. He can be cranky at times, but he likes to tell stories.
The Vault Dweller first meets him in the Old Town section of the Hub, where Harold 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 some tactical information on the deathclaw that lives near the Hub, which Butch Harris, leader of the Far Go Traders, requests the Vault Dweller to deal with, as he is losing his caravans to it.
Sometime between 2162 and 2241, a small tree (which he calls Bob, although he likes to joke that his name is Herbert) began to grow out of the side of Harold's head. It is an entirely new species of tree - unique and special, just like Harold.
After the destruction of the Master's Army, Harold left the Hub and became a traveler who visited various places around the wasteland. It's known that he traveled in Texas to Carbon where he had a sexual encounter with Carbon's prostitute, Ruby, and the ghoul-city of Los around 2208. He wanted to have a good time in Los after leaving Carbon but the arrival of super mutants and the loss of his hand, his toe and his eye in the city changed the deal. The Initiate helped Harold to recover his parts so he could leave the city before its destruction. Harold's travels ended in Gecko in 2238.
In 2241, Harold is a very old but still spry-looking ghoul. The Chosen One runs into Harold in Gecko, the ghoul town not far from Vault City. After the destruction of the Necropolis following Fallout, most of the ghoul population of the town migrated far to the north to form a settlement around an old nuclear power plant built by Poseidon Oil before the Great 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 and stupidly. Harold quickly took over from the well-meaning but inept leaders of Gecko and got the reactor into some kind of working order. When the Chosen One arrives in Gecko, Harold asks him to help powerplant problem.
The Chosen One still heard mention of Harold from time to time. Apparently, the tree growing from his head has gotten larger, and if rumors are to be believed, fruit is growing from it. The seeds are said to be remarkably tough, and several of them have taken root even in the most barren stretches of the wasteland.
The following is based on Van Buren and has not been confirmed by canon sources.
By the 2250s, Harold was ill. The tree in his head developed an unknown disease and was dying. Much to his surprise, Harold found himself affected as well. He didn't think he was going to die, but, he just didn't feel right in the head.
So, he set out on a quest for a cure. Naturally, he didn't have an easy time of it either. People just didn't seem to want a mutant around any more. But, Harold didn't let that stop him. He persisted in his quest until he came upon the Twin Mothers tribe, which originated from his home, Vault 29.
Much to his surprise the tribe took him in and accepted him as he was. He explained his quest to the tribal leaders and was told that they would consult their goddess. Days later, Harold was approached by the tribal shaman and given a potion to drink. It was a foul concoction, but it worked. Bob, the tree, got better and was happy again.
Harold couldn't let such a good deed go unrewarded, so he offered to help the tribe in any way that he could. They smiled at him, and thanked him, but declined his help. "The goddess will provide," they always said. Harold said that he would like to pay his respects to the goddess, and was taken to the tribal shrine to be granted a private audience. He wasn't really surprised when the projected image of a woman appeared before him, but he was taken aback when she told him where he could find her. His return to the Nursery was a moment of joy for Diana and a 'new experience' to him, as much of his early memories faded with age.
Harold traveled to the Nursery and spent considerable time there, He even considered settling down and spending the remainder of his day in the tranquility of the gardens. However, eventually he left and traveled even further east.
If the Prisoner gives Diana the information on the FEV and the New Plague from the Boulder ZAX, Diana will be able to create a cure to the New Plague virus in the form of a small fruit, by genetically engineering it from Harold's tree.
Harold eventually found himself overwhelmed by the growth of Bob, and he became rooted to the ground during his travels in the northern Capital Wasteland region. Here, he was discovered by several people who began to worship him as a god, forming a small and exclusive cult, known as the Treeminders. Bob began to blossom, and many plants grew in this area, which became green with life. During this period, Harold developed a mutation which enabled him to "see" the area around Oasis, through the very trees that came from Bob's seeds.
He would use his new found ability to bring in wanderers, all the while hoping one of them would put an end to his troubles- unfortunately, the Treeminders' strange ways, and insistence on using an intoxicating, potentially poisonous "sap" (as a "purification" ceremony) drove off most of the wanderers, many in worse mental shape than they were prior. His persistence may pay off should the Lone Wanderer come into view. On Harold's orders, the Treeminders can allow the Wanderer safe passage into Oasis.
If the Lone Wanderer meets Harold, he reveals his true identity and begs for a merciful death after being stuck in the same position for decades. Although commanding the Treeminders to perform questionable deeds for his own amusement provided some entertainment, it did not last. The other dwellers of Oasis either ignore or misinterpret Harold's wishes as a moral and spiritual test, much to Harold's chagrin. Thus, his only recourse is to ask the Lone Wanderer to take his life. However, two members of the Treeminders (namely, the leader of the cult and his wife) can be found arguing about Harold. The leader of the cult wishes for Harold's gift of plant life to be kept safe and confined to Oasis, while his wife wishes for Harold's gift to be shared and spread throughout the wasteland.
They could ask the Lone Wanderer to accomplish their own wishes, and the Lone Wanderer can decide whether to carry out Harold's wish of death, the cult leader's wish of keeping Harold's spreading growth confined to just the Oasis or the wife's wish to spreading his glory across the wasteland. Through granting Harold's wish, the Lone Wanderer's skin will be as hard as Harold's. If the Wanderer chooses to keep Harold alive, they can tell Harold that he has been given a gift and must stay alive to save the lives of others. Harold, finding a new reason to live, replies that he was selfish to want to kill himself, and then asks Bob if he agrees. Harold finally says that he is at peace with himself and, for the first time in a long time, Harold is finally happy.
To the untrained eye, Harold appears to be an ordinary ghoul. This is not so, however. Ghouls are the result of massive and/or long-term radiation damage to a human body; Harold is a product of the Forced Evolutionary Virus. Unlike most people who are exposed to FEV, Harold did not become a super mutant, but is the result of a unique combination of radiation damage from constant low-level environmental exposure, indirect exposure to FEV (it is unclear how Harold was infected with FEV as he blacked out for a while during his exploration of Mariposa) and a fair amount of random chance. Thus he's not a ghoul, and certainly not a super mutant. To quote Tim Cain, "Harold is special."
Furthermore, Tim Cain has this to say on the subject: "As for contact [with FEV], any contact at all will infect the subject, but the amount of contact determines the result. For example, I imagine Harold had some contact with the virus, but he was not fully immersed in it, so he became a different mutant than the Master's subjects. Full immersion, of course, is the preferred method of infection, as it provides the virus a large surface area for infection."
Another similar FEV-created, ghoul-like mutant is Talius.
Harold's unusual behavior is treating Bob as if the plant were actually sentient. Although at no point does the plant indicate that it can convey feelings, Harold certainly is not stupid, and perhaps only crazy in the humorous sense— yet he still insists on hearing out the plant's opinions on certain matters that he might otherwise take seriously. Given Harold's new found ability to "see" through the trees that are descended from Bob through the use of extreme concentration, it may not be much of a stretch to suggest that both are deeply linked in some unknown way, almost certainly as a result of FEV exposure. Because of his mutation, it is no surprise that Harold is afraid of fire.
Despite his appearance, Harold is a kind-hearted soul with a comical personality. He has made many friends throughout his travels, potentially including the Vault Dweller, the Chosen One and the Lone Wanderer. He is also good with kids, as evidenced by his friendship with Sapling Yew in Oasis, of whom he is particularly fond.
In Fallout, Harold's "Tell-Me-About" on vaults leads to his explaining that the vault he originated from failed, due to an overabundance of vault dwellers, and a lack of food to sustain their numbers. This actually corresponds to the description of Vault 27 in Fallout Bible, which is not canon material. This vault was to be deliberately overcrowded by twice the sustainable amount, as opposed to Vault 29 as it's described in Van Buren, not canon either. This means the official fate of Harold's home is the one he tells.
A group called "Harold's Cult" is mentioned in a piece of concept art by Adam Adamowicz, which was released after Adamowicz' death. It portrays three characters in clothing similar to that of the Treeminders.