I think that it is Zack. cuz (SPOILER) in the game, they said that the man in the pipe wuz found near Midgar. Zack wuz shot near Midgar. The man had a number and so did Sephiroth. And since Cloud was a failure clone, he didnt get a number. Zack must have been a sucess and got a number. Remember he wuz saying "ooooooooo auuuuuuuuuug" well, Cloud made the same noise so maybe when Zack wuz shot he wuznt blasted to pieces, he wuz just shot in the chest and a scientist stumbled upon him, overdosed him wit mako or sumthin and no hes doin what Cloud did in Mideel.

or maybe it's just me.