Overratedroth.

[Sephiroth]

If you don't accept the Compilation then you cannot complain about anything. The Compilation is Final Fantasy VII itself and not only Final Fantasy VII"-1". And even though there are some things which were changed and of course things like "Sephiroth wants to absorb everything like JENOVA" is more clear after Advent Children because in Final Fantasy VII-1 he obviously just wants to absorb Gaia most of his character was already developed in Final Fantasy VII-1.

And Sephiroth's overconfidence is understandable. He needs to have a "weakness".

- He is the most powerful character in the Final Fantasy VII universe (and so of the Final Fantasy X universe because we all know it is the same universe)
- He has the strongest will of all characters
- Because of his will he can partly control the lifestream - he is the evil part of the lifestream if you want to call him that - and uses the magic he can summon with the lifestream with such a destructive power no one else can because his willpower allows him to absorb the knowledge and memories in the lifestream and so of course he becomes stronger and stronger
- He is the best swordsman of all and uses a Nodachi like no one else can use it because Nodachis can only be used with the Iai-Do Style and he uses it like he wants it to use
- He can come back over and over again because of his will power combined with the ability of the Reunion

Et cetera.

With such power I would be overconfident as well. Of course he is proud of that power and Sephiroth himself says he thought he was something special since he was a little child. But experiencing why he is something special was the first surprise and then being defeated by Cloud was too much surprise during one week.