The Goron Merchant is a character from The Minish Cap. The Goron Merchant does not appear early in the game and will only appear after Link has successfully fused a Blue Kinstone with the Goron Digger found at Lon Lon Ranch. This will cause the Goron Merchant to setup shop in Hyrule Town where he will sell Kinstone pieces to Link.
Link is able to fuse Green Kinstones at various points in the game with the Goron Merchant. 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. The Goron Merchant is one of the many characters in which Link can make these Kinstone fusions with.