Deckard Cain

Deckard Cain is a recurring mentor within the Diablo universe. He serves as a background character for Leah's opening/ending.

Franchise lore
Deckard Cain, also known as Cain the Elder, is the last living human in the line of Horadric Mages. Appearing in all of the Diablo games, Cain provides the player with valuable advice for many quests. He is descended from Jered Cain, one of the founders of the Horadrim who imprisoned Diablo two hundred years before, and thus is schooled in the most obscure lore and history of the arcane.

Trivia

 * Cain is known for his Sean Connery-esque tone of voice.