Jenova

[Squall of SeeD]

JENOVA's important to the story for the following reasons:

Some thirty years before the main events of the game, Professor Gast of Shin-Ra Inc. discovered an apparently dead being in a geological stratum. He named the creature "JENOVA" and made an error in identifiying it as being the body of an Ancient (otherwise known as the Cetra).

He then conducted experiments in an effort to bring forth one with the abilities of the Cetra into the present age. His assistant Hojo and another assistant, Lucrecia, had conceived a child together. Hojo and Lucrecia offered up their unborn child and Lucrecia herself to experimentation. Believing JENOVA to be an Ancient, some of JENOVA's Cells were injected into Lucrecia's womb where they melded with the developing body of Sephiroth.

However, JENOVA was not a Cetra, but an offworld being that came to the Planet some 2,000 years before the main events of the game. It released a virus (possibly its own Cells) among most of the Cetra, it transforming them into monsters and driving them insane. A small band of Cetra who had eluded JENOVA united in an assault on the creature and somehow successfully sealed it away in the Planet.

In the meantime, the Planet had created five large monsters with which to battle against JENOVA. These were the WEAPONs. However, the Planet didn't use them once JENOVA was sealed away (for some reason) and put them in stasis, to awaken once again in the event that the Planet was in a crisis.

JENOVA's landing -- either with its own mass or that of Meteor -- caused the gaping wound in the Planet known as the Northern Crater, a wound that has not yet healed in the 2,000 years since JENOVA's arrival and confinement. According to Ifalna, the mother of Aerith, the Planet can never fully heal so long as JENOVA remains, and the being itself wasn't dead after being discovered by Gast and could reawaken at any time.

JENOVA's Cells were injected into folks aside from Lucrecia, by the way, including the residents of the original Nibelheim who survived Sephiroth's massacre there (all save Tifa, who Zangan took to safety), Cloud, Zack, and all members of SOLDIER. It is also through the JENOVA Cells that JENOVA/Sephiroth is able to control, manipulate, or otherwise influence certain People, whether it be outright control as in the case of the black-cloaked folks (the former residents of Nibelheim), manipulation (as in the case of Cloud), or slight influence (as in the case of the Junon Accessory Shop owner, a former member of SOLDIER who suddenly felt the urge to don a black cloak). The game suggests that one's self-awareness, or -- more accurately -- their acceptance of who they are is the determining factor in how suspectible they are to the influence being exerted over them.

It's possible -- and I would say likely -- that JENOVA itself awoke when Sephiroth was in Nibelheim and influenced his mind, causing him to lose his sanity there, and that it was influencing him throughout the game, with the two essentially becoming one, or -- more accurately -- Sephiroth becoming an extension of JENOVA's will, possibly even believing ideas he might have had were his own when they were just being fed to him.

That last paragraph is conjecture, mind you, but what is assured is that JENOVA is the unifying plot device of the entire game through which most of the main plot points are a result or are somehow affected.