Dumpy is an unusually tall dwarf refueling the stove within the Blast Furnace on Worlds 352, 355, 358, 386 and 387, provided 72,000 coins are deposited into the coffer near the bank chest every hour. Players can speak to him to learn about operating the furnace.
Speaking to him or his brothers, Stumpy, Pumpy, Numpty, and Thumpy, will reveal that they have a rare genetic condition that makes them unusually tall. The player points this out, but he will say that this condition benefits them by allowing them to efficiently operate the blast furnace.
The five dwarves here operating the furnace are only available on designated Blast Furnace worlds