A mule is essentially a side character that's only used for either their skills or inventory. For example, many people make HS mules (which is a level 81 priest) for the sole purpose of using it for HS. They don't train it at all after obtaining max HS. Another example is an SE mule. People level up a bowmaster or marksman to level 129 and stop training it afterwards (because by then it'll have max SE) , and they use it solely for its SE skill. Other common mules are SI, HB, and sometimes Haste mules. Other than skills, people make storage mules, simply to have more space to store their items such as ores and equips.
