He and Ares have a chequered history; Ares stole the Dark Chakram from him and gave it to Xena, which strongly angered Kal. He returned to him years later, after Xena had been ressurected from the dead by Eli and the Dark Chakram broken by Callisto. He hoped to get his hands on the Chakram of Light, which had the power to kill Gods. However, the Chakram could only be lifted by someone with a pure soul, since the Dark Charkram was absent from the altar and was therefore out of balance. Many died trying to retrieve the Chakram (including Eli's friend Calib).
Since they both had a common goal (to wield the power to kill Gods), they decided to put their differences aside and work together to lure an amnesiac Xena into the Chakram's temple to lift the Chakram of Light from the altar. However, when Xena did eventually restore the Chakram, she used the Chakram of Light to kill Kal, before fusing the two Chakrams together, which got rid of the power to kill Gods and created a brand new Chakram with the ability to split in half and act as two different weapons.
He was played by Antonio Te Maioho and is totally original with no parallels in mythology– the Latin war god was Mars, a less bloodthirsty version of Ares.