Vault Boy is the mascot character of the Vault-Tec corporation within the Fallout universe, appearing in their adverts, manuals, products and training films. He was also to appear in some issues of the Hell's Chain Gang comic of Hubris Comics, but because of the Great War, the series was never produced.
In the Fallout games, Vault Boy is used to provide an iconic representation of almost all stats (perks, traits, skills etc.) and items in later games available to the player character, being a generic representation of one's actions and survival, and also appearing in achievements and trophies for Fallout 3 and Fallout: New Vegas as well as the mascot of the Fallout series itself. In appearance, he is a young male cartoon character with wavy blond hair wearing a vault jumpsuit. He most commonly expresses a wide grin, but has been shown to make other facial expressions as well.
Source: The Fallout wiki page.