Mul gro-Largash is an Orsimer miner.
He works in Gloombound Mine in Narzulbur. He left Largashbur with his brother Gadba. Their plan is to get rich and then go back to Largashbur and take over.
He is the smarter one of the two brothers, as Gadba himself points out.
The only real interaction with Mul is if the Dragonborn kills his brother, Gadba. He will send three hired thugs after the Dragonborn, each with about 100 gold on them, some armor, and a contract.