Devil is one of the main antagonists/Bigger Bad in the Tekken fighting game series. He made his debut in Tekken. He is the Devil Gene that lives in the bodies of Kazuya Mishima and Jin Kazama. Simply referred as 'The Devil', he actually originates from Kazumi Mishima, the wife of Heihachi Mishima.
After Tekken 2, Devil split in two different entities: Devil Kazuya and Devil Jin. His original incarnation serves as Kazuya's devil form. His other half was able to reside in Jin Kazama's body 3 years before Tekken 3, as Jin was forced to inherit it from his father through the Devil Gene. Devil split from Kazuya after he was nearly killed at the end of The King of Iron Fist Tournament 2. According to Doctor Abel, the devil cells destroy the human cells and the Devil takes over the human body. Thanks to G Corporation, Kazuya learned to coesist with Devil and the two merged into one entity, and after Tekken 4, Kazuya always controlled his own emotions and is now able to use his Devil form at will. Jin apparently never learned to turn into Devil at will, yet he's been able to do it a few times, and it might be thanks to his Kazama blood.
In Tekken 7 it is revealed that Kazuya inherited the Devil Gene from his mother Kazumi Mishima at birth. A scene in the opening of Tekken 3, portraying a ghostly figure branding the arm of Jin with a mysterious tattoo that leads many fans to theorize that Devil would later possess the body of Jin, after the disappearance of his mother, Jun Kazama who had protected her unborn child from it 15 years ago. However Jin inherited the Devil Gene from his father at birth.
Devil is a power-hungry, unforgiving entity who is motivated by power above all else, and enjoys causing pain to other people, suggesting a sadistic nature as well. He displays no remorse for the damage that he causes to others in his quest for absolute power and control over the world. He is also extremely arrogant, as he believes that his power is unmatchable. He appears to be very cunning and is ruthless in battle. He also very manipulative, such as attempting to lure others who are cursed by an evil entity like Jin to succumb to its power through merciless fear and pain, in attempt to gain theirs as his own. Unlike Devil Jin's dislike for Jin Kazama, Devil actually likes Kazuya, since the latter gave up to hatred and malice.
After being thrown into a mountain ravine by his father, Heihachi Mishima, Kazuya Mishima gained a evil energy known as "Devil Gene" witch made him extremely powerfull. Kazuya, driven by anger and hatred entered the tournament to kill his father, Heihachi. The power of Devil allowed Kazuya to finally defeat Heihachi in the final round in Tekken.
Since Kazuya met Jun Kazama, another entity was created within Kazuya known as Angel which represented all that was good within him.
Kazuya announced the King of Iron Fist Tournament 2 in Tekken 2, Heihachi Mishima beats Kazuya and the Devil Gene inside him causes transforms into Devil. Devil is still defeated by Heihachi and thrown into a volcano. Parts of his entity and Devil Gene were destroyed by this.
The part of Devil that fled attempted to inhabit the body of Kazuya's and Jun's unborn child Jin Kazama. Jun fought this part of Devil off but it succeeded fifteen years later, after her apparent demise at the hands of Ogre, when it branded the now teenage Jin with a mark on his left arm, possessing him (as shown in the Tekken 3 prologue). Since Jin still had the Angel side of his mother in him, Devil and Jin were not one being like that of Kazuya and his Devil.
During the climax of the third King of Iron Fist Tournament Jin was shot in the head by Heihachi. Revived by the Devil Gene within him, Jin reawakened as Devil Jin and made quick work of Heihachi and the guards that accompanied him. Jin has been able to use the power of Devil to transform into Devil Jin ever since.
In Tekken 4 Kazuya returned to retake the Mishima Zaibatsu. Devil's real intention was to find Jin Kazama to take Jin's Devil Gene in order to be complete and monstrous again.
In Kazuya's ending of Tekken 4, Devil consumes him when he goes with his father and see his Jin; So Kazuya dislikes the devil gene by inside Jin. In Kazuya's non-canonical ending, Devil manages to regain his former glory after he recovers his fragment from Jin's Devil and takes full control of Kazuya, resulting with himself and Kazuya become one. In the actual ending Jin beats Kazuya and his grandfather, Heihachi. He partially transforms into Devil Jin and almost kills Heihachi, but a vision of his mother Jun Kazama stops him. Jin flies away and Devil Jin takes over his body for weeks after these events.
In Tekken 5, Kazuya and Heihachi fights some of Jack-4s. Later, Kazuya transformed as Devil Kazuya and flies away. Since completely unifiying his power with Devil, Kazuya seems to have become even more consumed by his evil side, taking over G-Corporation and attempting to take over the world with evil intent in his eyes.
Project X Zone 2
Devil will appear in the upcoming sequel to Project X Zone as a transformation for Kazuya.
Namco X Capcom
Devil Kazuya makes an appearance as a boss in Namco X Capcom.