Ruby is a non-canon female red Ninja character who appeared in Mortal Kombat: Defenders of the Realm. Ruby was very similar to the characters Tanya and Jade. Like Jade, she was a friend of Kitana and came from a realm which Shao Kahn had invaded, which was called Hinpar. Ruby was notorious for being a traitor, much like Tanya in the games, betraying all her allies for the likes of Shao Kahn. It is assumed that Ruby may have been the killer of Jax's grandfather, as she states that she "took care" of him before he died.
In the Defenders of the Realm series, Jax meets Ruby, who is secretly working as one of Kahn's agents. She tries to find out the location of the Earth warriors base. Jax then finds himself captured by Ermac's forces and is held captive by the Emperor, where he was tortured by Kahn's Shadow Priest. Jax's allies came to his rescue, but were shortly outnumbered by the Emperor's troops, and they too were captured. It seemed that their destruction was inevitable, but at this 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, but Ruby decided that her place was in Outworld. After this, Ruby was never seen again.
Federation of Martial Arts
Ruby is not seen, but plays an integral role in Jax's story in Mortal Kombat: Federation of Martial Arts. She was said to have been "spending time" with Jax (alluding to a secret romantic tryst), but she was killed. While Jax was looking for her killer, he was blamed for her murder and put on trial by the Elder Gods. Jax's footprint was shown to have matched up with the wound on Ruby, and he was sentenced to immediate execution. However, it was revealed that a shape-shifting Black Dragon ninja was the true culprit, and he was executed in Jax's place.