Bob started to grow on Harold's head sometime between 2162 and 2208, after Harold's accident at Mariposa Military Base, where he was exposed to Forced Evolutionary Virus (or F.E.V). At the start, Bob was a plant without roots who supposedly lived with the aid of Harold's mutated body, but as Bob started to grow into a fully sized tree he developed roots that caused Harold to be rooted to the ground in the Capital Wasteland region.
Bob and Harold were later discovered by several people known as Treeminders, who started to worship them as gods. Soon after, Bob started to blossom and many plants grew in the area which would become known as Oasis.
Harold believes Bob is his friend and sometimes speaks to him as if he was thinking aloud. However, in Fallout 3 Harold admits that Bob is now too much and therefore Harold wishes for the player to kill him.
Children of Bob
The Children of Bob are the seeds planted by Bob. They are found scattered around throughout the Core Region. A whole forest has been formed by the Children of Bob. This forest is the oasis in the Capital Wasteland inhabited by the Treeminders. Harold is able to see past the Oasis using a mutation that allows him to access the Children of Bob.
Bob spends his life together with Harold, although this may not have been something they would have chosen. Harold will sometimes jokingly refer to Bob as Herbert.