Leah is kind of a central NPC in Diablo 3, and she seems to have an extensive history with Deckard Cain, often calling him "Uncle."
However, the game is pretty light on details as to who she is, precisely. It's clear she's not ordinary, for instance:
2/3rds of the way through Act 1, she shows some awe-inspiring, but uncontrolled supernatural powers.
I'm not super familiar with Diablo lore on the whole, having only played Diablo 2 for a few months many years ago. Who is Leah? What's her role in the Diablo saga?