Cosplay as Sephiroth takes a lot of skill, and not just anybody can pull off the correct look. To start with, you should always make your own custom outfit, that way you don't end up looking like someone else's Sephiroth. You should use a professional seamstress to help with the basics if you are not good with Sewing. A pre-made costume could be okay for the average fan, but people can tell that it is often somewhat plastic looking. A true Sephiroth costume will look like a the outfit of a true warrior.
Remember also to have an almost exaggeratedly long sword. Sephiroth's sword is as long as he is tall, and difficult to wield. Yours should be similar, but can be made out of lighter material so that you can wield it properly. The wig needs to be extremely life-like, so use lots of Sephiroth Pictures to ensure that yours looks correct.
A wing is optional, as Sephiroth does not have it in every scene. If you are trying to pull of a Kingdom Hearts Sephiroth, then a wing is recommended. The Kingdom Hearts version of Sephiroth has the red trim, and he has his one black wing throughout Kingdom Hearts.
Final Fantasy VII
Final Fantasy University
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