while you are right that the true evil of FFVII is Jenova but Sephiroth was basically a human form of Jenova as far as the evil went. I mean you cant really Jenova to be considered the evil when it is in a fucking glass test chamber thing. Jenova didnt really do anything but sit there and look "scary". Sephiroth was just a total badass he was uber strong and from what i could tell he didnt need materia he simply could use "Mako" energy to cast spells.