From Diablo Wiki
Leah is one of the main NPCs in Diablo III. She is a regular source of conversation and quests in town, as well as a giver and rewarder of quests. She even ventures out into the dungeons with the player, on at least two occasions in Act One.
Leah's true parentage, as revealed early in the game is the daughter of Adria the Witch and Aidan, King Leoric's older son who was the warrior in Diablo I, and in the official lore became the host of Diablo in Diablo I. Many fans suspect that Leah's pedigree and involvement in the plot will lead her to fall victim to a similar fate in Diablo III.
- Warning: Leah is a main character in Diablo III, and this entry contain story spoilers.
Leah is first encountered very early in Diablo III. She is the second NPC players speak with, after Captain Rumford at the front game of Tristram directs the new character to speak with Leah in the Slaughtered Calf Inn. There Leah explains what she knows about the fallen star and urges players to help find her lost Uncle Deckard. Soon afterwards, she accompanies your character on their journey to Adria's Shack, opening the locked gate out of Old Tristram.
Leah joins the player in adventures later in Act Two as well, as shown in Blizzard's Inferno gameplay teaser movie.
It's not yet known if Leah travels with the Caravan into later acts of the game. It's been much speculated that she has a large roll in the ultimate events of the game, perhaps even acting as Diablo's human host, but this is not confirmed. See the sheablo article for details.
Prior her role being confirmed by the beta testing, there were several clear indications that Leah was an early, instrumental NPC.
Leah and Deckard Cain were first seen in town in screenshots of New Tristram from August 2010. Both are standing in a shadow near the very top of the screen in the images; a cropped version is seen to the left, with the levels turned up to make them more visible.
Prior to this official appearance, Leah made a sneak preview in an April Fool's screenshot with the Archivist. She was not initially recognized in the screenshot, partially since she's facing away from the camera, but her costume corresponds very well to the concept art of Leah seen in the Blizzard Museum at BlizzCon in August, 2009.
A fan realized this and pointed her out in September 2009, and a confirming comparison of the images can be seen to the right. 
Jay Wilson spoke about Leah in an interview from Gamescom, August 2010: 
Leah has appeared in all of the Diablo III cinematics yet released.
- Cinematic: WWI 2008. Leah narrates this movie, which introduced Diablo III in June 2008.
- Diablo III Introduction cinematic. Leah appears in this cinematic, which player will see at the start of the full game. It establishes the world lore, as well as introducing Leah and Deckard Cain, and setting up the story and events of Act One by showing the fiery meteor crashing down into Tristram Cathedral. It ends with Leah's infamous "Uuuuuunnnnnnnccccccccllllllleeeeeeee!" wail.
- The Black Soulstone cinematic. Leah is the star of this cinematic, which will be seen between Acts 2 and 3 of the game. In it she is working on Cain's Journals and pondering the Black Soulstone, before falling into a dream or trance in which Azmodan and his demonic armies are first seen.
Now that more details are known, it's clear that Leah is the daughter of Adria the Witch and Aiden, King Leoric's older son, who (in the official lore) was the Warrior class in Diablo I, and ultimately the Dark Wanderer who became the human host of Diablo in Diablo II. Leah's ultimate role in Diablo III is not yet known, though in the minds of many fans, she is the leading candidate to become the human host for Diablo in the game. See the sheablo article for more details.
- I likewise found it odd that there seemed to be only elderly people in the village, with the exception of one beautiful young woman whose father was quite insistent I keep my distance from her. Though he was rude on this point, I found him to be a rather sociable sort once convinced I was not interested in his daughter. He introduced himself as Pablo DeSoto
This is clearly not the case, now that we know Leah's origin as Cain's adopted daughter.
This damned power I have... Uncle Deckard said it came from my mother. He tried to teach me how to control it, using his Horadric training, but it never really helped. He never gave up on me, though. He was a great man.
Uncle Deckard and I traveled all over the world in my youth, looking for lost lore and artifacts. It was so exciting, crawling around ancient temples, digging through forgotten crypts...
I feel fortunate that Uncle Deckard raised me the way he did. The things I learned from him... But now I don't know. I see what his obsession with his quest has cost him... has cost us...
About the Temple
The Drowned Temple was once known as the Sarceum Emporis. It was the most sacred of their temples, a site where legend claims angels would come to impart their wisdeom to the nephalem.
Leah has only been indirectly seen in screenshots, thus far. She made an appearance with the Archivist in an April Fool's shot, and is seen in shadows at the top edge of the screen in several of the early Caravan screenshots. She has appeared more often in cinematic shots and pieces of character art.
- All Leah images in the gallery.
With Cain in cinematic concept art.
|Characters & NPCs Import. Chars Rumoured Diablo 3 NPCs Diablo 2 NPCs Diablo 1 NPCs Minor A-K Minor L-Z Fanmade|
|Diablo III's Possible NPCs|
|NPCs of Diablo II||
Achilios - Adria - Akara - Albord - Aldric Jitan - Alkor - Anya - Arkaine - Asheara - Assur - Atma - Augustus Malevolyn - Baerna - Banagar - Benjin - Brek - Buyard Cholik - Camylle - Celestia - Cornelius - Darrick Lang - Diomedes - Drognan - Edgewulf - Elora - Elzix - Emilye - Fara - Farnham - Fautzin - Galeona - Garda - Gheed - Gillian - Gorst - Greiz - Griswold - Guthwulf - Hagan - Halbu - Hanos Jeronnan - Hargo - Hays - Hratli - Hrothwulf - Humbart Wessel - Hunfrith - Hyram - Icharion - Jace - Jamella - Jerhyn - Jodas - Juris Khan - Justinian IV - Kabraxis - Kara Nightshadow - Karybdus - Kashya - Kentril Dumon - Kethryes - Korlic - Kreghn - Ku Y'leh
Lam Esen - Larzuk - Linarian - Lysander - Malah - Mat Hu-Ring - Meshif - Mirakodus - Mordecai - Natalya - Nefriti - Nellia - Nihlathak - Nor Tiraj - Norrec Vizharan - Ogden - Ord Rekar - Orlif - Ormus - Oskal - Palashia - Pepin - Qarak - Qual-Kehk - Riordan Nesardo - Sadun Tryst - Salene Nesardo - Sardak - Sarnakyle - Septumos - Simon - Talic - Taramis Volken - Tilgar - Tobio - Toko - Tylwulf - Vischar Orous - Warriv - Wirt - Wulfgar - Xazax - Zagraal - Zayl - Zorea