Devil
From the Tekken Wiki
Devil (sometimes called Devil Kazuya, as there is Devil Jin) is a character and one of the main antagonists in the Tekken fighting game series. He is the final boss for every character in Tekken 2 with the exception of himself. He represents Kazuya Mishima after he has succumbed to the effects of the "Devil Gene". Though there are supposedly multiple "devils" within the series, only the "devil" which inhabits Kazuya is referred to as just "Devil".
[edit] History
After being thrown into a mountain ravine by his father, Heihachi Mishima, Kazuya Mishima sold his soul to an entity of frightening power known as Devil in return for the power to overthrow his father. Tekken 2 suggests that Kazuya's soul was not fully consumed by Devil and that the two are,Tekken 3 in fact, separate entities. For example, the character profile for Angel states that she represents Kazuya's good side. Also, Kazuya's Tekken 2 ending, depicts him battling against Devil. However, during the timeframe of Tekken 4, Devil and Kazuya apparently act as one.
However, choosing both Kazuya and Devil in Tekken Tag Tournament, and then tagging either character into the match results in the morphing of one into the other. (Tagging in Devil while playing as Kazuya results in Kazuya morphing into Devil, and vice versa.) Before the fight, whoever was chosen first will enter the match alone.
Furthermore, Tekken 4, it is stated that Devil and Kazuya were once separate entities before Kazuya unknowingly managed to "unify the two powers into one". In Kazuya's Tekken 4 ending, Devil (while in Kazuya's body) explains that during their battle against Heihachi 20 years ago, he lost a part of himself when Heihachi attempted to kill him (his other half was stored within Jin Kazama). In addition, in Kazuya's Tekken 5 Story Mode introduction, he morphs into Devil to escape Honmaru. Devil has not officialy been a playable character since Tekken Tag Tournament, but he made a brief cameo in Tekken 5, as well as some of his body parts being able to be customized on Kazuya, such as the Devil horns, Devil wings, etc.
