His father, Heihachi Mishima, threw him off of a cliff as a young boy to see if he could climb back up, as the lion cubs do. Kazuya survived, and during his arduous struggle back up the cliff, he swore to get revenge against his father. This hatred is the dominant force in his life. His entire existence has been spent preparing for this one act of violent. As Kazuya was going to die an evil spirit flowed through him."Will you help me?" Kazuya asked? "Yes, in exchange for your soul." the Spirit spoke. Kazuya answered yes and the spirit bonded with him. 20 years later a research team called WWWC had been researching for a boy fallen off a crag which was Kazuya. The first researcher to find Kazuya was Jun Kazama.

(Source: Tekkenpedia)