Sephiroth is the primary antagonist of Final Fantasy VII. There is a brief section of the game where he s in the same party as Cloud and this is during the flashback sequence when the party arrives in Kalm.
There isn’t too much to know in the way of strategy for Sephiroth when he joins your team. Sephiroth is desperately overpowered during the sequence at Mount Nibel and the Nibelheim Reactor.
That being said, Sephiroth is also the final boss of the game. If you’re looking for more information about Sephiroth as it relates to the final battle of the game at the end of Disc 4, check out the Sephiroth walkthrough section.