Note

Who or what is the Kingdom Hearts Sephiroth, really? In KH2 much is made of him supposedly being the dark part of Cloud's heart, yet in the game where Zack features, Birth By Sleep, he says he wants to become a hero "like another," and it has been officially confirmed that Zack meant Sephiroth. This line tells us so much, both that KH Sephiroth is a real person and not literally the darkness of Cloud, and that somewhere along the way, he, like his FF7 counterpart, was a hero. What happened? There is so much we don't know, so many blanks to fill in. I will be playing KH Sephiroth similar to how I wrote him in stories some years ago, as a very strange man who claims to be Cloud because he once fell to his darkness and sees that Cloud is following the same path he did, and that instead of trying to push Cloud to accept the darkness as it seemed, he is actually trying to get Cloud to accept things about himself that he refuses to accept, as well as to use reverse psychology to get Cloud to decide he's better than to fall to the darkness and to choose the light instead.

Also, his wings are blue, not black. And another thing Birth By Sleep tells us is that Sephiroth had them as a hero and everyone knows it. Just the presence of one large feather is enough to signal he's there.

What happened to Zack? Well, judging from these vital scraps of information, I'm going to say Sephiroth observed Zack's behavior and his desire to be a hero, and when Phil still wouldn't help him, Sephiroth swooped down, took Zack with him, and started training him himself. As for where Zack is now, that has yet to be revealed. But it's vital to this Sephiroth's backstory.