1) According the the online translation of the Ultimania Guide, it was definately Sephiroth. Regardless of the fact it makes more sense that it was Jenova.

2) Did Sephiroth have signs of going bad before the Nib. incident? None that I noticed.

3) What did you think of his personality? Before the Incident, a quite normal guy who seemed pretty cool. After words a power obsessed man who likes to mess with clouds head from time to time. Nothing special.

4) I can't remember whether it was said in the game or not, but why did the clones go directly to Sephiroth, instead of Jenova? Because he controled Jenova, and thus was able to control the Clones.


Really there isn't that much to Sephiroth to analyze.