That's debatable
The truth is... Jenova is Ultimecia.
Yep, FFVII's 'background villain' is actually Ultimecia.
There is IRREFUTABLE PROOF in both games.
Jenova can survive travel through space. Thus, Jenova could get from one world to another. If the worlds of FFVII and FFVIII are in fact the same world, then this is fine too - Jenova can survive for millennia, buried in solid rock.
"But Jenova was destroyed!" you all cry. Actually, Jenova's body has the ability to re-form when it's dismembered - even when it's apparently dead.
Both Jenova and Ultimecia are female, and both want to attain God-like power and rule over an entire world.
Jenova has the power to change her form. Ultimecia transforms, as well. Jenova can alter her appearance, gaining the appearance and voice of other people. Jenova and Ultimecia look nothing alike, which proves that they are one and the same, just using different forms.
This is utterly, completely, irrefutably, incontrovertibly, undoubtedly, undeniably, undisputably, indubitablly, unquestionably true.
Coz I sez so.