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The Past
Well, it seems like Sephiroth didn’t die five years ago, even though Cloud managed to throw him into the Lifestream under the Nibelheim reactor. We can only assume that he floated from there to Northern Cave where he rested in his shell of mako until Cloud gave him the Black Materia.
The most common idea is that he’s one of Hojo’s Sephiroth clones, which seems to be borne out by the numeral ‘I’ we know is tattooed on him. However, we also know that most “clones†are just normal people infected with Jenova cells, but perhaps in sufficient number, they could affect a physical transformation: we do see “Sephiroth†make himself look like Tifa at Northern Cave, and tell us that “The ability to change one’s looks, voice, and words, is the power of Jenova."
We know that he was a relatively normal person until his false discoveries that his mother was Jenova, and that she was an Ancient. Sephiroth knows that what he learned in Nibelheim is far from the truth, so the reasons for his crusade must have changed somehow. My thought is that Sephiroth’s confusion and sorrow over what he thought he discovered in the Shinra mansion, combined with his proximity to Jenova, allowed him to be influenced into his later actions — the strange effects of the Lifestream may have compounded this, especially as he was carrying Jenova’s head close to him. We see a very similar chain of events occur through Cloud, giving Sephiroth the Black Materia and falling into the Lifestream, except that it’s Tifa who gets to talk to him in there rather than Jenova.
Whatever the reason, Sephiroth’s goal is to cause a huge disruption in the Lifestream by having Meteor strike the Planet, and to be at the focal point of the Lifestream’s energy at that point, allowing him to soak it up and become a god of sorts, along with Jenova. Is it really his ambition, or hers? Sephiroth is the main villain, but it’s also clear that he’s a victim and a puppet of Jenova and Shinra, even more so than Cloud. The fact that he doesn’t even speak after the first Northern Cave encounter (other than in flashback) certainly doesn’t make things any clearer. Nevertheless, manipulated or otherwise, Sephiroth is dedicated to this goal, and it’s this that our heroes must ultimately stop.
The present
After the fall of Jenova, the ShinRa Company crippled, and Migar as a desolate wasteland. Midgar and the people are at peace however, a pure source of evil resurrects in the grounds of the Northern Crater. Sephiroth rises once more, he now seeks out the once that had laid him to rest, without Jenova controlling him. Sephiroth is a renegade SOLDIER, with his return, If shinra were to find out that Sephiroth is alive and no one has control of him, Shinra would want him as a product once again. Sephiroth would defend himself and kill anyone that try to claim him as a puppet, he will defend his indenependence at all costs. And if any advisary must destroy Sephiroth, he will defend his right to live.
Time has passed, the people and grounds have changed since his silent demise. Sephiroth lurks in the shadows of Midgar, the people that have seen him, fear him. He wanders around without a mission or a quest, if he is find the enemies of his past, and the hate still stands. Without any type of emotion or remorse, he will fight to the death. As of now, he wanders to see what the future holds for him.
