Kazuya hasn't been relevant since Tekken 4 and isn't who i think of when i think of Tekken.
When i think of Tekken = Heihachi, Nina, Yoshimitsu.
Relevant? What you mean in the story?
Has Nina EVER been relevant to anything? Storyline wise she's just a hired gun (and in 6 she was hired by Kazuya, was she not?) Nina is maybe the least interesting character in the game with consistently the worst costumes