Use the JENOVA editor on qhuimms site to put Sephy into your party. that simple.
...Slightly off topic, and, I appologize, but... I've had it.
What's with you people who want to play as Sephiroth?! He has no limits, no animations for half your commands, you can't control him anyway, and the whole point of the story is to find him and stop him!
..though i suppsoe there is some comedic value to all that:
Cloud: "We must find Sephiroth and stop him!"
Tifa: "yeah!"
Sephiroth:"...sure, whatever you say."
Cloud: "Shut up, Sephiroth, I'm not in the mood for your sarcasm! Now c'mon..."
*sigh*, people....