Zauz is a blacksmith who resides on Zauz's Island. It's near the top-center. This trip is optional. The first time Link meets him, he tells him that the two of them will meet again. Within his house is a sign Link can use by drawing it in the Temple of the Ocean King.
Later in the game, Zauz is instrumental in helping Link through the forging of the Phantom Sword. To do this, however, he requires Link to go out and find the the Pure Metals in order to create the sword, which has the power to defeat Bellum.