Also known as Cain the Elder, Deckard Cain (voiced by Michael Gough) is the only character aside from the title character, Diablo, to appear in every installment of the series. Through in-game interactions in Diablo, the player learns that Cain is the last surviving member of the Horadrim, an ancient order of mages charged with trapping the Prime Evils within Soulstones and guarding them thereafter. Cain acts as advisor and guide, accompanying the player's character through each act of the games and providing a great deal of the lore to be found in-game. Despite appearing very old already in Diablo II, Cain has been revealed as a returning character in Diablo III, which takes place twenty years after the second game, and he is expected to be a somewhat darker character.
Source: Wikipedia, "Deckard Cain", available under the CC-BY-SA License.