Alleria Windrunner is the eldest of the three Windrunner sisters. She was a Ranger in the Alliance army during the Second War. It was during the Horde's razing of Quel'thalas' borderlands when Alleria heard a rumor that her sisters had been killed. In a thirst for Vengeance, she formed a group of Rangers to hunt down the Bleeding Hollow clan, a group of Orcs who had formerly eluded capture.
Shortly after, she joined the Alliance Expedition and followed the Orcs through the Dark Portal to their homeland of Draenor. After Khadgar closed the portal to prevent any further transportation of the Horde, Alleria was cut off from her homeworld, and has not been seen since.
Ranger Captain Alleria Windrunner
Renowned Troll Hunter of Quel'Thalas. Lead Scout and Intelligence Agent for the Alliance Expedition that marched into the orc homeworld of Draenor. Presumed deceased.
Your heart flew straight as any arrow upon the wind, sister. You were the brightest of our Order. You were the most beloved of our kin. - Sylvanas Windrunner - Ranger General of Quel'Thalas
Finally, Alleria is mentioned in a questline, starting in the Ghostlands, where you are asked to deliver a necklace gifted to Sylvanas from her.