Rauru (Ocarina of Time)
Rauru is one of the Six Sages in Ocarina of Time. After Link wakes up from his seven year slumber he will encounter Rauru in the Sacred Realm. Rauru will proceed to explain why Link was trapped in there for seven years, and also tell him that while he slept Ganondorf had free access to the Sacred Realm and consequently to the Triforce. He then gained the power to take over Hyrule. Finally, he reveals Link's destiny: to become the Hero of Time. Then he will tell Link to find the other Sages and add their power to his own, giving him the Light Medallion.
Later in the game it is Rauru who tells Link to return to the Temple of Time after awakening all of the Sage. Also, at the end Rauru helps seal Ganon away in the Dark Realm.
Theory
It is believed that the owl, Kaepora Gaebora, is in fact the sage Rauru. Kaepora Gaebora watches out for Child Link, and coincidently isn't seen much of after Link turns into an adult and Rauru takes his place.
Other Appearances
Wind Waker
Even though Rauru doesn't make an appearance physically in-game, a stained glass window of him can be seen in Hyrule Castle, along with the Stained Glass of the other Sages.
Majora's Mask
If Kaepora Gaebora is indeed Rauru in a different form, then we can add Majora's Mask to its appearances. However, this is unconfirmed.