Sareena is a demon, she is from the Netherrealm and was an assassin for Quan Chi, but turned against him.

Her young and beautiful appearance hides her true form of a hideous demon. In the past, she required Quan Chi's magic to sustain her human form, but since recently escaping the Netherrealm, she has managed to find a way to remain beautiful without him, that’s when you notice Sareena in the Netherrealm’s lava caves.

”You… you’re the young Earthrealmer back on Outworld..”