He is the first of the seven sages to awaken, revealing himself as the Sage of Light. Rauru is extremely knowledgeable, knowing much about the land and history of Hyrule, although he can also be a bit long winded.
Link first meets Rauru as the Owl Kaepora Gaebora. Throughout link's adventures as a child, Kaepora gives link advice and sometimes provides shortcuts around the world map. This is the only time Rauru is seen outside the sacred realm.
When Link enters the Sacred Realm, he is required to rest for seven years in order to grow older so that he would be able to properly wield the Master Sword. Rauru kept him here for all this time, and explained to him what his obligation was - to first awake the rest of the sages and use their powers to thwart Ganondorf. Following this he gave Link the Light Medallion (Rauru himself is known as the Sage of Light).
After sending Link off he awoke the other sages and requested that he find someone waiting for him at the Temple of Time. There he found Zelda. Doing so, however, revealed to Ganondorf the location of Zelda, and he quickly kidnapped her. With this, Link was required to save her after Rauru told him to do so. He and the five other sages opened the door to Ganon's Castle for Link, and in the end the seven sages (which included Zelda as well) sealed Ganon away. In the ending ceremony, six of the seven sages would overlook the celebration on Death Mountain, Rauru not included.