Mileena is one of the adopted daughters of Shao Khan. She eventually underwent genetic experiments at the hands of Shang Tsung. These mutations mixed Mileena's Edenian genes with those of the Tarkatan. This transformed her into a hybrid with incredible strength and ferocity. Mileena would eventually come to rule the outworld when her father, Shao Khan, died. However, her reign was not as long as his thanks to a coup set in motion by some of Shao Khan's conquered realms.
She is cruel and unforgiving. Ready to murder for the sheer pleasure of it. She very much exists as an antagonist to Kitana.