His name is Cobal. He is a member of a highly civilized race of moles called the Mogma. All Mogmas have a deep and abiding love of treasure.
Cobal is first found close to the Earth Temple, where Link overhears a conversation between Cobal and his treasure hunting partner, Ledd, about the broken key required to enter the temple.
As many Mogmas' names are derived from minerals, it is likely that Cobal comes from the element Cobalt.