Actually, Kazumi existed since the 1st Tekken although she's merely mentioned in name. And yes, it is implied that Kazumi is the originator of the Devil gene. Harada himself already hinted this since last year. Tekken 7 itself is meant to celebrate the 20th anniversary of the series.
A photo of her was also shown in the 1998 Tekken anime (non-canon story):
Yep, they even reveal the silhouette of what looks like the devil form of Kazumi in the trailer and in the Bana Passport cards above, so she might reappear again in the current timeline... although still no word whether she'll be playable or not. I do hope she'll be
