Auru is one of the members of "The Group" in The Legend of Zelda: Twilight Princess, and could be called the oldest out of them. He is fairly knowledgeable about the desert and the evils that it used to house (or, indeed, still does), and appears to have known the sages who guard the Mirror of Twilight at some point; aside from knowing that they once tutored the young Princess Zelda, it was apparently they who first told him of the mirror and its location. It could be inferred that he himself has had links to the Royal Family at some point: aside from his name bearing a strange resemblance to Rauru (which could also be said to be on account of his wise and elderly nature), the emblem of the Triforce is visible on the front of his clothing.
You first meet up with him in southwestern Lake Hylia at the top of a tall lookout tower. He will tell you that he believes the evil currently plaguing Hyrule is related to the desert- specifically, the Arbiter's Grounds, a prison formerly used to house "the worst criminals this land (Hyrule) has ever known..." also, as it happens, the resting place of a cursed mirror supposedly used to send criminals sentenced to death directly to the underworld. Whilst he tells you that the road leading to the desert is impassable, he helps you get there through use of Fyer's cannon- the memo he gives you instructs Fyer to send you to the desert for free, a request he cannot refuse due to Auru's saving Fyer's life a long time ago. It is in such a way that Link gains access to the Gerudo Desert and, eventually, the Mirror of Twilight. After completion of the Arbiter's Grounds, Auru can be met at any time in Telma's Bar.