::Limit Break, The best part??::

[Squall of SeeD]

Guys, you're all wrong in some way, though between all of you, you have this stuff right.

1) The creature that Kadaj summoned is Bahamut Sin.
2) Aerith isn't an angel.
3) Kadaj, Yazoo, and Loz are Remnants of Sephiroth, which is to say that when his body was blown apart, the JENOVA Cells he melded with reformed into these three guys and they were seeking to reunite. They are not black-cloaked folks who didn't die at the original Reunion, as the bodies of these three are completely composed of JENOVA's Cells, whereas the bodies of those guys (like Cloud) were obviously not.
4) It was really Sephiroth that Cloud fought. He and JENOVA are one, thus, he is a virus. He exists in all of JENOVA's Cells because he is JENOVA.