I don't know if anyone has seen this yet, but it is pretty cool. In Japan they released a book called "Hand in Killer 7" and it explains the truth behind everything that happened. It also tells how the Killer 7 were able to transform into each other, please check it out. I won't spoil anything.

I think you can find this as an FAQ at Neoseeker, or GameFAQ's. I actually came across it at the IGN Board for the game, and maybe you should check there for it.

You can find the translation of this Japanese only book at this link.

Hand in Killer 7