There's a reason for ghosts to disappear all of a sudden - they don't want you to notice them, that's why they disappear.

http://img390.imageshack.us/img390/9439/z2wa1.gif

This picture, you see both lighter and darker substances (both in half) on the Ghost area, that's how you started having doubts but since I have a very bright monitor, I could see the area very clearly.

- Why is the Ghost next to Cloud?
- Why is the Ghost's hair black?
- Why are the Ghost's shoulders/arms naked? Just like Cloud. Zack doesn't wear full suit covering shoulders and arms. Just like Cloud. He wears almost exactly the same as Cloud, both of them also have the same swords, go figure. I'm a fan of Zack, I have seen his sights quite a lot recently.

http://img206.imageshack.us/img206/9648/10492230rt9.gif

The answer is Zack, obviously.

Unbelievable.