Vault Dweller

In general definition, it's a human who's living in one of 122 vaults.

When capitalized, it's the protagonist of Fallout, he saved humanity from the threat of Super Mutants invasion by killing Master.

History

It is believed by many that the Vault Dweller comes from Vault 13. Many tried to find the Vault Dweller's home but no one succeeded. Vault 13 is hidden somewhere to the west of Shady Sands, currently called New California Republic.
The Vault Dweller was sent out from Vault 13 on a quest to find a Water Chip. If he would fail his Vault would run out of water. During that task he saved Tandi (current president of NCR) from the hands of Khan raiders. The Vault Dweller also gathered alarming information about a Super Mutants army. He eventually found a water chip somewhere in Necropolis.
Soon after returning to his home, the Vault Dweller was sent out once again. This time his task was to eleminate the Super Mutants threat. The Vault Dweller destroyed the source of Super Mutants in Mariposa military base and defeated their leader - Master.
After all he had done for his Vault, the Vault Dweller was exiled by his own people. He travelled to the north and founded a village, somewhere in the mountains west of Klamath. According to the Fallout canon, one can not see Arroyo on the world map of FOnline: 2238.