Honestly, it's not clear due to how badly the story was told. To me there was no "main" villain. One could argue both Kazumi and Kazuya because Heihachi fought them both. But that's be too simplistic an answer.

Well it focus on both heihachi and kazuya as villain protagonists with kazumi as the bigger bad

Women can't be villains. Don't you watch Disney? 😏

Uncle Gav is a certified yapper fr