The great thing about Cloud is, at the start of the game, he's a hard man, ice cool and a bad ass to boot. Then we see his fits, but this is downplayed a bit. Next, Sephiroth controls him. How is he able to do this? And then the real events of Nibelheim with Zack being there certainly had me puzzled, so I daren't think what Cloud must have thought. The first time round, I thought that Cloud knew full well what happened at Nibelheim, and he was lying about it. After replaying the game several times again, I began to realise what had really happened - aided by the flashback of his and Zack's escape. Cloud had confused himself with Zack, and this is what Sepiroth used to control him.
Yet when Cloud discovers the truth, it doesn't destroy him. Yes, he has a bit of Soul Searching, but the massive illusion that he had believed in so strongly suddenly shattered in front of his eyes, coupled with the fact that Aeris had so recently been snatched away from him, Cloud must have felt his world falling to pieces. Yet still, he picks himself up, and marches on to face Sephiroth - he knows he is the only one to do it.

Cloud = great.