If you play Dissidia, the two can fight. Both can defeat each other.
Kefka.
That could be an opinion but I'd have to say Sephiroth. It is an opinion and it's usually between Sephiroth and Kefka. Personally, I say Kefka because while was sane and compassionate until he became Jenova's puppet, Kefka was working of his own free, deranged will. Kefka just likes destroying things, Jenova/Sephiroth wants to kill off humans.
No... Only Chuck Norris can defeat Kefka
probably sephiroth.
Try it in dissidia.
No.
Maybe, this can be tested in FF Dissidia but i am certain kefka would win.
When you defeat Sephiroth, Tifa gives you the Fenrir Keyblade.
Definetely.
Sephiroth's father is Hojo, a famous scientist who is in Final Fantasy 7 and Final Fantasy 7 Crisis Core, and his mother is Lucrecia Crescent, who worked with him on the Jenova project.
No. Just No. Sephiroth is to purely awesome.
Yes.