Kazuya Mishima's origins are marked by childhood trauma when his father, Heihachi Mishima, throws him off a cliff at age five for being weak and overly shaped by his mother's kindness. This brutality follows Heihachi's earlier murder of Kazuya's mother, severing his primary emotional bond. Kazuya survives the fall only through a demonic pact with the entity Devil, exchanging his soul for power. This leaves him scarred and ignites his lifelong quest for vengeance.

Sixteen years later, consumed by hatred, Kazuya enters the King of Iron Fist Tournament to confront Heihachi. His personality is coldly ruthless and isolated, rejecting any interference with his mission. During the tournament, he encounters Jun Kazama, an Interpol agent investigating Mishima Zaibatsu. Unbeknownst to Kazuya, Jun witnessed his fall as a child and possesses his mother's locket—a symbol of his lost humanity. Kazuya initially dismisses her attempts to dissuade him from vengeance, yet their interactions trigger fragmented memories of their childhood meeting.

Kazuya's inner conflict intensifies as the tournament progresses. He proves merciless in combat, nearly killing opponents like Michelle Chang who obstruct his path to Heihachi. However, Jun's persistent appeals challenge the demonic influence within him. During the final confrontation, Kazuya fully succumbs to Devil's power upon learning Heihachi orchestrated his trauma to cultivate darkness. Overpowering Heihachi, he prepares to execute him. Jun intervenes, returning the locket. This act reawakens Kazuya's suppressed humanity and memories of his mother's compassion, enabling him to expel Devil's control. He rejects vengeance and spares Heihachi.

In the resolution, Kazuya rescues an unconscious Jun during the island's destruction, signifying his return to inherent goodness. His ultimate fate remains unresolved, though an epilogue implies he fathered Jun's son, Jin, before vanishing.