The eyes on search system is balls.
I don't know if this question has ever been asked or answered in any thread/forum related to Final Fantasy VII or the characters therefrom.
But recently looking at a picture of Cloud from Kingdom Hearts, I was struck by memories of scenes from the Crisis Core trailers. It always had me dumbfounded that he had such an ugly wing on his back. Then I realized it . . . Cloud is a demon. A planet-born anomaly of mass potential with an undeniable fate driving him toward the destruction of the "angel" Sephiroth. The characters Angeal and Genesis both end up with an angelic wing like Sephiroth's.
Sephiroth and Genesis were both SOLDIERs created by ShinRa via Mako and Jenova cells. Angeal strikes me as bearing a greater similarity to Zack as the experiments don't really seem to affect them. However, Angeal eventually reacts to the experiment where Zack does not.
It seems that Sephiroth sees Zack as a common human: a neutral being with no potential. But you can see it when Sephiroth plunges his sword through Cloud: he sees deeper potential in this creature who seemed to not be affected by the experiments but continues to defy him through sheer willpower. He says it's impossible and asks him what he is.
I say he is a Demon. Cloud . . . is a demon.