Vault 77 never appeared in any of the games, instead being featured in the official Fallout webcomic series.
While many of the other vault experiments involved overpopulation and crowding, Vault 77 was inhabited by just one man. For months, the man bashed on the vault door, assuming that more vault dwellers were on their way. Soon, he discovered that he had been left a crate of puppets, which he later gave names and personalities. Becoming quite mad due to the prolonged isolation, he invented a whole life based around the puppets, even becoming embroiled in a murder mystery involving the ‘death’ of one puppet.
Eventually, the Vault 77 dweller managed to escape, and entered the wilderness. After being captured by slavers, he managed to kill both of them while wearing a ‘vault boy’ puppet, earning him the ‘Puppet Man’ nickname.