Master

The Master, was the Brains behind the Mutant Army. He was born in Vault City as Richard Moreau, where he was a doctor, but was exiled from there for murder, which he presumably did not commit. He changed his name to Richard Grey and went South to a town called Hub. There he became a leader in a team of scientists involved in an expedition to the North, to an apparantly abandoned Military Base. Among them were Harold, which became a good friend of Richard Grey during the course of the expedition. When they got to the Mariposa Military Base, they found it to house the Forced Evolutionary Virus vats, and the government had been doing experiments here before the war broke out.

Richard Grey accidently fell into one of the FEV vats. Harold and the rest of the surviving members of the expedition decided Richard had to be dead. They left with a great feeling of guilt upon their shoulders and escaped the base in fear of suffering the same fate, but Richard did not die.

He was one of the very few, and lucky, humans to be modified by the FEV without having his brain and and intelligence reduced, but rather it increased instead. Richard's mind and body were supposedly melded togheter with many other personalities his diaries seem to indicate, and shortly he felt stronger and more capable than before. With his new confidence he decided to start unifying the wasteland all as perfect as he had become, and he went about in his search for humans to dip into the FEV to create his muntant army. Unfortunately, the results weren't as great as with him; it just made a bunch of dumb big soldiers.

Harold was the only survivor and escaped to the Hub, it seems; the rest of the scientists got killed by guarding robots on the way out.

Related Holodisks
Richard Grey Audio Diary

The Master in Fallout games
The Master appeared only in the first Fallout game. He was mentioned in all other games, though, and his being previously called Moreau and being exiled from Vault City was revealed in Fallout 2.