The unplayable boss named alongside NANCY-MI847J in Tekken 6 is Azazel.
Azazel is Tekken 6’s enormous demonic final boss. In the story, astrologist Zafina warns Lars Alexandersson and Raven that Azazel is an ancient evil sealed inside a temple in Egypt. Jin Kazama deliberately causes worldwide conflict to generate enough negative energy to awaken it.
NANCY-MI847J serves a different role. It is a giant combat robot encountered as an optional bonus boss in arcade play, while Azazel is the climactic opponent in the main story. Neither is a normal selectable fighter in standard gameplay, which is why both are commonly grouped as Tekken 6’s unplayable bosses.
A common mix-up is Jinpachi Mishima, the final boss of Tekken 5. True Ogre belongs to Tekken 3, and “Unknown” is associated with Tekken Tag Tournament rather than Tekken 6. Azazel is also central to later lore involving the Devil Gene and Zafina’s efforts to contain its power.