Kazuya Mishima is one of the main characters in the Tekken series. He appears in all Tekken games except Tekken 3, in which he only made two cameo appearances. Although he is seen as the main protagonist and hero of the first Tekken game, Kazuya became one of the major antagonists of the series ever since Tekken 2. Most importantly, Kazuya is the son of Heihachi Mishima and Kazumi Mishima, the possible love interest of Jun Kazama and the father of Jin Kazama. He’s also the half-brother of Lars Alexandersson, the adoptive brother of Lee Chaolan, and the grandson of Jinpachi Mishima.
At five years of age, Kazuya finds out his mother died at the hands of his father and attempts to get revenge against him. However, Kazuya is defeated almost immediately and gets thrown off a cliff by his father, who claims that his son is weak and would have to survive and climb back up if he was to be his true son as well as his successor. Kazuya barely survives the fall, but is able to climb back up the cliff due to his Devil Gene activating and causing his devil powers to awaken. In the non-canon Tekken: The Motion Picture, this was explained as Kazuya, on the brink of death, making a deal with with the Devil in order to survive to solely murder his father. In the NTSC-U/C (North American) manual of Tekken 2, it was also said that “there is a rumor that Kazuya has made a deal with the Devil,” but in canon sources this has never been directly said to have happened. It seems that as the series has progressed, the canon explanation for his devil powers has instead become that his near-death experience simply caused his latent powers granted to him by the Devil Gene to activate. It is revealed that Kazuya was born with devil’s power through his mother.
After climbing back up the cliff, Kazuya was raised by Heihachi. This is evidenced by Kazuya’s profile in the first Tekken, which states that, since his father is a rich businessman, he doesn’t need to worry about money, which seems to suggest that Kazuya remained a part of Heihachi’s household. It is also stated that Kazuya and his adoptive brother Lee Chaolan were raised from childhood as rivals, and that Heihachi adopted Lee because he was a skilled scrapper living on his own in the street, meaning Lee would have to be older than 4 years old when he was adopted. This means Kazuya was raised by Heihachi even after he was thrown off the cliff, since he and Lee were raised together. Before the events of the first King of Iron Fist Tournament, Kazuya participated in martial arts tournaments or unofficial fights around the world. In one of these fights Paul Phoenix fought him to a draw. When the first King of Iron Fist Tournament was announced, the 26 year old Kazuya took the opportunity to get back at his father, setting the stage for the first game.