Kazuya assumes his devil form when performing certain moves, as listed below. It is merely an aesthetic effect, although some of these attacks activate Special Zoom or have armor. Kirby can also take the appearance of Devil when using Devil Blaster after copying Kazuya.
List of moves
Devil Kazuya is simply Kazuya that activated what is known as the Devil Gene. The exact specifications of what the Devil Gene is and how to acquire it have never been fully explained, though it is hereditary and the children of a carrier are likely to get it. It is not a traditional gene in the scientific sense, but more of a curse. The gene and the carrier share a parasitic relationship, where a spirit known as a "devil" resides within the body of the carrier and sometimes takes them over. Aside from a more violent, malignant personality, the carrier gains darker skin, horns, wings and a third eye that can shoot a laser beam. The prerequisites for activation are also unknown, as the devil can seemingly take over at any time, even to infants still in the womb. However, most devil activations have occured during episodes of intense stress and desparation from the carrier.
It is theorized that the demon Azazel is connected to the Devil Gene in some way. Many have claimed that both Azazel and the gene share a source of power, implying that Azazel gave the gene to the first carriers when he ruled the earth thousands of years ago.
There are three confirmed carriers of the Devil Gene. The first is Kazumi, Heihachi's wife and Kazuya's mother. Her maiden bloodline, the Hachijō family, has possessed the gene since ancient times and passed down the gene through the generations. This implies her family also has a connection to Azazel. She ends up using her powers to fight and kill Heihachi, as she feared he had become too powerful and became a threat to the world. Heihachi ended up killing her out of self defense and to rid the world of the demon he thought was the love of his life.
The second carrier is Kazuya. After he witnessed his mother killed by his father at age 5, he challenges Heihachi to a fight out of vengeance. He is quickly defeated and thrown off a cliff by Heihachi, to both test if Kazuya truly has the gene and if he could be a worthy successor to the Mishima Zaibatsu. The gene is the only reason Kazuya survived the fall, and he would spend the next several decades training and mastering his devil powers out of revenge. He loses half of his devil at the end of Tekken 2, when Heihachi throws him into a volcano. This saga reaches a climax in Tekken 4, where he overpowers the devil within him and takes control of his powers, and he is then able to turn freely turn his powers on and off at will. During the climax of Tekken 7, he is shown to possess an even stronger version of his devil form, distinct from the usual one he uses that also appears earlier in the story.
The third carrier is Jin Kazama, son of Kazuya and Jun Kazama. The other half of the devil that Kazuya lost ended up trying to insert itself into the unborn Jin, but Jun managed to ward it off temporarily. However, the devil finally managed to merge with Jin 15 years later. Both halves of the devil attempt to assimilate each other during Tekken 4, but are unsuccessful. Devil speculates that the Kazama blood within Jin is preventing the merger. Jin eventually activated the gene and seemingly is able to turn it on and off at will like his father.
After throwing Kazuya off a cliff, Heihachi wondered if he possessed the Devil Gene. He then had another son with an unidentified Swedish woman, named Lars Alexandersson, as a test subject. After learning that Lars did not have the Devil Gene, Heihachi abandoned Lars and his mother, and he could not recognize Lars as his son when meeting him again during the events of Tekken 6. Initially a simple Tekken Force soldier, Lars went rogue and staged a coup d'etat against the Zaibatsu. The only people confirmed to know of his lineage is himself and Kazuya, who have a silent agreement to stay out of each other's way.
Heihachi also experimented with synthetic vitro-fertilization to create a human strong enough to withstand the devil gene. The only confirmed subject to survive is Steve Fox, created from the harvested eggs of Nina Williams while she was cryogenically frozen. He was also abandoned after the experiments proved fruitless, and he spent his life training as a professional boxer. He despises the Zaibatsu and has tried many times to reconnect with his mother, but Nina refuses to acknowledge him as her son.