History Behind The Warrior covers the (retconned) past of Devil Kazuya

In the Tekken universe, the “Devil Gene” was originally a supernatural element in the story. When Tekken 4 moved in a more sci-fi oriented direction, Kazuya’s internal “Angel vs. Devil” struggle became a genetic mutation. This is just one of the aspects of Devil Kazuya that has been modified or entirely retconned, as History Behind The Warrior latest video explains.

If you’re interested in what’s changed for other characters, such as Ogre and King, you can check out other Tekken lore videos on his channel.

Source: History Behind The Warrior