Ruby is a red kunoichi (female ninja). She was a friend of Princess Kitana and she originated from a realm called Hinpar which fell victim to Shao Kahn's tyranny.
It is assumed that Ruby may have been the killer of Jax Briggs' grandfather, as she states that she "took care" of him before he died.
Jax meets Ruby who is secretly working as one of Kahn's agents. She tries to find out the location of the Earthrealm warriors' base. Jax then finds himself captured by the warriors of Ermac's forces. He is held captive at the emperor's castle where he receives numerous tortures by Kahn's Shadow Priests.
Jax's allies come to his rescue but they are outnumbered by the Emperor's troops and are captured. It seems that their destruction is inevitable. At that moment, Ruby decides to help the Earthrealm warriors escape by leading them through a hidden exit.
Jax offers Ruby a chance to go along with him. Ruby decides that her place is in Outworld in order to fight for her old homeworld. They bid farewell after Ruby sheds a tear.