A mule is a character that is created solely for usage of its inventory and warehouse slots. Mules are often needed, but stays an option, for players who choose to avoid from purchasing Cash Items, since the limited inventory slots given to a character might become a hassle, especially when doing dungeon runs which reward the player with valuable drops.
With the use of the Leveling Rewards event, a player may create a Level 20 character and keep it in a town, in close proximity of the Mailbox, the Exchange Deacon, and the Warehouse Manager in that town.