*SIGH* He wasn't even a Cetra. He was a lab experiment. Had he known that Lucrecia was his mother and that he was nothing more than a human(considering his genetic code) with some special powers granted to him by the cells of a space monster that craves destruction. Isn't it funny that the very that DESTROYED the Cetra is convincing Sephiroth that HE IS a Cetra.

This ends all. Sephiroth became curious about his past. He sought information. He found it in the undertground lab. He read about how Professor Gast THOUGHT that Jenova was a Cetra. He concluded that Jenova was a Cetra and that he was her biological son.

He concluded on the evidence provided in the books that he was the last Cetra. The information was false however. Jenova merely strengthened the thought. Making it seem the truth. Even if Cloud and the others had tried to tell Sephiroth, Jenova wouldn't let him listen.