Diablo II: Deckard Cain
Deckard Cain is the most popular and important NPC in all three Diablo games. He serves as a mentor to the hero and lore keeper for the game, driving the story and giving more context to the hero's journey.
Of course, he also provides his service of identifying items for free, as long you rescue him in Tristram in the quest The Search for Cain.
He will help your defeat the forces of evil, but only if you stay a while and listen.
Sisters to the Slaughter
The Horadric Staff