Yuga is the main villain and final boss of Samurai Shodown 64 and Samurai Shodown 64-2: Warrior's Rage, also playing an important role in the PSX-only Samurai Shodown Warrior's Rage.
Yuga is a powerful sorcerer born on Makai, the demon world, who wishes to merge the human and demon worlds to conquer them all. For this end, Yuga sought to create an army of slaves to do it's bidding through an elaborate plan spanning various years. Yuga entered the human world twenty years before the revival of Shiro Tokisada Amakusa, stealing various babies from their mother's wombs and granting them magical powers. When they all reach adult age, Yuga comes before them and performs a puppet show, successfully brainwashing them to serve him.
One of his slaves is Shiki, whom Yuga sends to seek a male counterpart needed for it's full resurrection. Shiki seeks Haohmaru, which Yuga had chooses as the perfect male for the ritual, but she is unable to bring him to her master. Haohmaru eventually confronts Yuga and defeats it, destroying it's physical form and releasing Shiki in the process.
One year later, Yuga resurrects and continues with it's plans. In order to retrieve Shiki, Yuga creates a clone of Asura, a warrior from Makai which it had imprisoned, and sent the clone after Shiki. However, not only the real Asura broke free and proceeded to hunt down Yuga, the clone ended falling in love with Shiki and betraying his master. The two Asuras then join forces and destroy Yuga's physical form once again.
Unknown to them, Yuga's spirit survived, possessing Shiki and Asura's daughter, Mikoto. For years, Yuga lived as Mikoto's devil side, which eventually caught the attention of Oboro, who entices her to become a member of the Razor Trio. Mikoto is finally confronted by Yuda, the fused form of both Asuras. During the confrontation, Yuga fully possess Mikoto and initially defeats Yuda, but this makes the two Asuras to awaken inside Yuda, who exorcises Yuga from Mikoto, finishing it off for good.