Quote Originally Posted by TheAbominatrix
Why would he only keep certain memories? And how would he get those memories. Zack is shot and killed in the field, not in a lab, so how would Cloud get those memories? And if he got his memories, why would he only get ones that fit convieniently into the story? He doesnt 'get' memories for Zack, he's told things by Zack, maybe just in an effort to keep Cloud from giving up and falling asleep (which can be fatal if you're sick/wounded), and he uses those memories as his own.
He also just knows things about Zack from working with him. Cloud didn't have to be told about Nibelheim, for instance.

I wouldnt say that JENOVA controlled Sephiroth the whole game... I say Sephiroth is working with Jenova...
Replay the flashback, especially the part where Sephiroth goes insane. Sephiroth "found out" that Jenova, an alien, was his "mother" from Hojo's notes. However, that's all he found out.
Then, later, he has this half-brained theory that he and his "mother" are Cetra, and the humans are traitors. Where'd he get that from? Jenova.
It's quite simple to believe that Jenova took control of him when he went insane by manipulative lies meant to get him to actually believe in her.