Master Smith is Link's grandfather in The Minish Cap. He lives in a house in South Hyrule Field with his grandson Link.
At the beginning of the game, Princess Zelda sneaks out of the Hyrule Castle just to meet Link and tell him that there is a Picori Festival and that she wants Link to go there together with her. Master Smith permits Zelda to take Link to the festival with her and he asks Link to do him a favor. He wants Link to deliver the sword that he has forged for the minister to be presented to the winner of the sword-fighting tournament. When Vaati causes trouble over at the Picori Festival, he breaks the Picori Blade and transforms Zelda into stone. Furthermore, Master Smith, Minister Potho, Link and King Daltus meets up in the throne room in Hyrule Castle and decides that Link sets out on a journey to recover Zelda and re-forge the Picori Blade. Master Smith then gives Link the Map of Hyrule so that Link doesn't get lost on his quest.
After entering Castor Wilds for the first time, an optional Kinstone fusion opens up with Smith. Link can fuse Red Kinstones with Smith, causing a Treasure Chest to appear in the Eastern Hills near Eenie and Meenie's house. The Treasure Chest contains an empty Bottle.
Link is able to fuse Green Kinstones at various points in the game with Smith. As it is a Green Kinstone fusion, it is not set to a specific Treasure Chest, but instead, one of the many possible Treasure Chests. There is one random Red Kinstone within the game that opens up a location in the Hyrule Castle Garden leading to a Piece of Heart, as well as a random Blue Kinstone that opens up a tree in North Hyrule Field leading to a Fairy Fountain. Smith is one of the many characters in which Link can make this Kinstone fusion with.