Kingdom Hearts II Show me your strength

Sephiroth, THE most popular villain in Final Fantasy has two appearances in Kingdom Hearts, he is in both the hardest boss (until Terra and the Data battles were added in KHIIFM+) in both games. I was just wondering if he is supposedly Cloud's Heartless or just his Dark side.

I think the best version would be that Sephiroth and Cloud's heart became intertwined when he was killed by Cloud because the Jenova Cells (In KH version Jenova is the ultimate Heartless) are in Cloud whenever Cloud uses the darkness Sephiroth returns.