Liu Kang traditionally protects Earthrealm in Mortal Kombat.
Earthrealm is the human world in the series’ cosmology and is repeatedly threatened by invasions, tournaments, and attempts by rival realms to conquer it. Liu Kang is usually portrayed as one of Earthrealm’s greatest champions, first as a Shaolin monk and later, in different storylines, as a powerful protector or divine ruler.
The conflict between Earthrealm and Outworld forms the foundation of the early games. Mortal Kombat tournaments are often presented as a way to determine whether one realm can invade or dominate another under the rules established by the Elder Gods.
Outworld, Netherrealm, and Edenia are separate realms with their own rulers and histories. Liu Kang’s relationship to Earthrealm changes dramatically in later timelines, but Earthrealm remains his traditional home and responsibility.