Kazuya is Heihachi's son and Jin Kazama's father in the Tekken series.
Kazuya is Heihachi's son and Jin Kazama's father in the Tekken fighting game franchise. He was originally the protagonist of the series but he would turn into a villain later, having been corrupted by his alignment with the demonic entity known as Devil.
One of the original Tekken characters from 1994's arcade release of Tekken 1, Kazuya and fellow character Paul Phoenix have been described as Tekken's version of Ryu and Ken. A rivalry between one warrior wearing white and another wearing red.
Originally a protagonist in Tekken, Kazuya's soul would become more corrupt over time due to the Devil within him. In his early appearances he had a mutually beneficial deal with the Devil, who would live inside Kazuya, taking over his body in order to transform it into a more powerful form, Devil. At the time, he was simply referred to as "Devil" as it was the demonic entity itself who was in control of the transformed body while Kazuya's mind laying dormant for the time being.
Kazuya did not mind selling his soul to Devil for his chance at revenge against Heihachi, as it was Devil who saved his life, allowing him the opportunity. Eventually Kazuya took on more villainous characteristics and a desire to rule the world. He completed his revenge in Tekken 1 and became the Head of the Mishima Zaibatsu (Empire).
Kazuya was presumed dead after Tekken 2, he had been defeated by a returning Hehachi and thrown into a volcano, Devil abandoned him to escape the same fate. Yet he re-emerged in Tekken 4, 17 years later rejoined with Devil. Kazuya at this point is now 49 years old and has had scientific experiments performed on him in order to unlock and control Devil's power for himself.
This was completed through a method of unifying Devil's power with his own, making Devil a part of his being. Thus from this point on (and in consistency with his son Jin having a Devil form, Devil Jin) any time Kazuya transformed he is referred to as "Devil Kazuya."
When Kazuya was a child, Heihachi threw him down a chasm, citing that only a true heir to the Mishima Zaibatsu (Empire) would survive and climb back up. The fall was too much for Kazuya, who had a gaping wound across his chest, his desire for revenge the only thing keeping him alive. This was seen by "Devil" who offered to save his life and give him demonic power to exact his revenge in exchange for his soul.
Kazuya defeats Heihachi at his own tournament and throws him down the same chasm that he was thrown down as a child. Through Devil's influence, the Mishima Zaibatsu becomes even darker under Kazuya. This gains Kazuya a considerable amount of enemies. Kazuya decides to finish them all in one swoop, announcing the 2nd King of Iron Fist Tournament.
At the tournament, Kazuya meets and becomes lovers with Jun Kazama. They have been said to have been drawn to each other through a mysterious force, this being the work of Devil, seeking to leave Kazuya and inhabit Jin, fully corrupted from birth. Part of this plan involves Devil placing half of his power in Jin.
Kazuya however is defeated by a returning Heihachi and thrown into a volcano. Devil escapes and attempts to posses an unborn Jin Kazama but is defeated by Jun Kazama. Luckily Kazuya is recovered by a company known as the G-Corporation and Devil returns to him after Kazuya is revived.
Heihachi's plan prior to Tekken 4 is to create a supreme being under his command, part True Ogre (Tekken 3 boss), part Devil. With Jin, who recently unlocked the dormant half of Devil's power missing, Heihachi seeks to claim Kazuya's body when he comes across pictures of Kazuya's deformed body.
He however does not know that Kazuya has been long since revived and is alive and well. When Kazuya foils his plan Heihachi lures out Jin and Kazuya by announcing the King of Iron fist Tournament 4. During the tournament, Kazuya successfully uses a method of unifying Devil's power with himself, essentially making them one being. He does not succeed, however, in defeating Jin and claiming the other half of Devil's power for himself.
At the end of Tekken 4, Jin leaves a defeated Kazuya and Heihachi, who are promptly attacked by the G-Corporation. Kazuya, seemly betrayed by them, briefly teams up with Heihachi to survive the assault of dozens of Jack-4 units. He then blindsides Heihachi, leaving him to be killed.
Kazuya then enters Tekken 5 upon realising that his grandfather Jinpachi is alive and the sponsor of the tournament. But after learning of the members of G-Corporation who betrayed him, Kazuya leaves long before the latter stages. He kills the executives responsible and takes control of the company himself, gaining hero status when he uses it to oppose Jin Kazama's Mishima Zaibatsu which has declared war on the world.
In truth, Kazuya simply doesn't want anyone ruling the world but himself.
As per his early role in the series, Kazuya is the protagonist of the Tekken anime film. His role in the 2010 live action film of the same name is that of pure evil, having raped and attempted to kill Jun Kazama (as opposed to the canonical video game story in which they became lovers).
Unlike his previous film roles, Kazuya will be based directly on his video game self in the upcoming CG film Tekken: Blood Vengeance. It is set in the Tekken 6 era with screenplay by Dai Sato, known for his previous screen work on Ghost in the Shell: Stand Alone Complex, Ghost in the Shell: Stand Alone Complex - 2nd Gig and Cowboy Bebop. Kazuya goes to war with his son Jin, both heads of powerful military corporations.