Devil first appears in Tekken as an alternate costume for Kazuya, who's partially overhelmed with his hatred by the demon. Devil is a playable character in Tekken 2, where he is the final boss and bigger bad of the game. He makes another appearance in Tekken 3, in a scene that happens 4 years before the game, where half of him inhabits Jin's body by heritage. He returns as a main antagonist in Tekken 4, but he is not a playable character and his meeting with Jin causes the last identity problem on Kazuya. From Tekken 5 onwards Kazuya and Devil act together as a single antagonist and Devil acts as Kazuya's "evil side" which is visible from his red eye since the fourth game. Devil's powers are not available with Kazuya in the fifth and sixth games. Devil is a playable character in Tekken Tag Tournament and his form is playable in Tekken Tag Tournament 2 and Tekken 7.
The Devil Gene is ancient and the demon Azazel claims to be its creator. Somehow the monster alien Ogre seems to have a relation with the Devil Gene and demons of Earth. 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.
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. Like the loud Devil Jin, he loves destruction, yet he is more calculating, subtle, and colder. 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. His most hated enemies have been Heihachi, Angel, Jun Kazama, and Jin. Devil's current main goal is to take back his half from Jin and restore his full power.
In Kazuya's story in Tekken 2, Kazuya can actually fight against Devil in the final boss battle, instead of Heihachi, Jun, or Angel. The part of the story is not part of the canon, yet fans believe that this alternate event might happen inside Kazuya's mind because Devil still has Kazuya's physical traits after leaving his body. However, Heihachi's presence in the ending would not be explained.
Although in the last 3 canonical games Kazuya and Devil get along together and Kazuya claims to control himself, some scenes and moments raised doubts from the fans about whoever between the two is really in control of the body and if Devil is actually Kazuya's real master, with Kazuya not being able to realize how evil he has become. For example, Kazuya is remembering happy childhood memories with his grandfather Jinpachi Mishima, who respected and loved, but since his devil cells are merged with his own, his happy mood suddently change into a sadistic evil one. It is believed that despite G Corporation's experiments, Devil's cells might be always the superior ones over the human cells.
After being thrown into a mountain ravine by his father, Heihachi Mishima, Kazuya Mishima gained a evil energy known as "Devil Gene" which made him extremely powerful. 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, it was believed that another entity was created within Kazuya known as Angel which represented all that was good within him. However, Namco has confirmed that Angel is not a part of Kazuya.
Kazuya announced the King of Iron Fist Tournament 2 in Tekken 2, where Heihachi beats Kazuya. During the gameplay, the Devil Gene inside Kazuya causes him to transform into Devil, but this part is not included in the canon. Kazuya, who is suffering identity and conscience problems, is 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 and fled to a life of isolation in the nature with her son.
But Devil succeeded anyway fifteen years later, after Jun's apparent demise at the hands of Ogre, when it branded the then teenage Jin with a mark on his left arm, possessing him (as shown in the Tekken 3 prologue). Since Jin still had a bigger good side of 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. From that moment, Devil has been slowly consuming Jin's body. Jin has been trying to get rid of his demon entity ever since that day. He (mistakenly) blames the cursed Mishima bloodline for this and hates Kazuya for his woes.
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 Heihachi and sees Jin in the desecrated Hon-Maru temple, which is infested by an evil spirit. Before Kazuya can have any reaction upon seeing Jin's chained, Devil takes over and reveals his plan and long wait to Heihachi. After Devil deals with Heihachi in the Hon-Maru, he tries to take back his half from Jin's unconcious body. Much to Devil's surprise it does not work and he blames the Kazama's pure blood from Jun's side. In that very moment, Kazuya fights Devil to regain control of his body, as he learned with G Corporation's help. A shocked Devil asks Kazuya what is he doing. Then both personalities decide to deal with Jin by killing him and take his power from his body. However, Kazuya never "devours" Devil, as the demon cells can't be destroyed by human cells: the G Corporation and Mishima Zaibatsu experiments could only go for the merging of both cells. Ever since this very moment, Kazuya and Devil became a single entity and one mind. Devil still likes Kazuya and now that the two are one share a single goal. Back than, before the events of Tekken 6 Kazuya had no interest in killing Jin if the latter wouldn't give him problems, but his Devil side was seeking his power back. 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 his demon cells destroying Kazuya's human cells for good. In the actual Tekken 4 second part of Kazuya's 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.
Still now, Kazuya's real enemy is Heihachi Mishima, which he succeeded to kill him in Tekken 7, while Devil's main enemy is Jin Kazama, the last threat they must eliminate and absorb Jin's Devil Gene.
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.