i heard sephiroth is supposed to be a experiment but my friend tells me he turns into a dark angel and angeal and zack turn into angels too, but why do they only have one wing?

The events in crisis core explain it, and sephiroth is a experiment

expanation:

Angeal was part of Project G - his mother, Gillian Hewley, was injected with Jenova cells before she gave birth to him. His mutation later reveals itself in the form of one white wing.

sephiroth has jenova cells and his mutation gave him his wing.

Zack gets his wings after he dies,(gift of godess my guess).

You know, this is why I hate additions onto already perfect game. They will spoilt the game to the max.

Was it really necessary for SQUARE to make Sephy into an experiment just to explain why he has one wing? They should just leave things as they were (in the original game).

i know the information from the story already, i just wanted to know why do they only have one wing?

an aspect of half demon thing

As cool as the wings looked (Especially on Genesis I think) Sephiroth is THE One Winged Angel! It's one of the complaints I'd have with CC. Too many wings!!