Cosplaying as Sephiroth requires a whole lot of information on how you are supposed to cosplay as lord sephiroth. Remember:
* Keep your hair long - Sephiroth's hair goes all the way to his knees... yours should be as long if you wish to remain true to Sephiroth's image.
* Keep your shoulder's shiny - Your 'metal pouldrons' (technical term) should be kept shiny, and should be made of metal.
* Have a long sword - You must have a really long sword (minimum 60") to Cosplay as Sephiroth, otherwise just go as one of the others (Kadaj, etc) from Advent Children. Don't besmirch Sephiroth's greatness.
* Have a custom outfit - You shouldn't look all plasticy, you should have spent a ton of time and energy getting your costume perfect.