Kazumi Mishima, Heihachi's wife and mother to Kazuya, is seen monologuing as if she is speaking to someone, asking him to stop Heihachi in her behalf when she meets her fate. Then we are brought to the present, where we see the father and the son face off on top of a raging volcano, with the camera zooming in on Heihachi as he sees a vision of his son transforming into Devil, and then he sees another vision of another Devil entity, who is most likely Kazumi.
The next scene goes back in time and practically confirms this fact as Kazumi talks about how the young Kazuya had inherited her powers, alluding to the Devil Gene. She is also aware of Heihachi's own power and fears what he can do to Kazuya and to countless others, so she takes it upon herself to stop the man she loves.
We have been treated to a lot of non-canonical comedy bits from the previous Tekken games, and I appreciate that they are upping the ante with their dramatic presentation. Let's look forward to what Tekken 7 brings forth, given that it will be running on the Unreal Engine 4.
Also, check out the character concept art for Kazumi and Lars.
(And by the looks of Kazumi's hairdo, I can say that Kazuya sure takes a lot from his mom, not only his Devil powers.)