Prospector Percy is the discoverer of the Motherlode Mine. Players can trade him golden nuggets for soft clay packs and components of the prospector outfit, as well as Coal bag and Gem bag. To reach him, you will need a pickaxe to mine the blocking roads leading to him.
By talking to him and asking him if there's any other things you can unlock, he will reveal that for 100 golden nuggets and a level 72 Mining level (not boostable) you can use the upper section of the Motherlode Mine. For 200 golden nuggets you can get the larger sack upgrade, increasing the capacity of the sack at the end of the cleaning machine from 81 to 162.
Percy is the solution to the hard anagram clue: COPPER ORE CRYPTS. He will then give you a challenge clue; the answer is 12.