Along with Zack, a promising member of SOLDIER, and Cloud, a bright Shinra escort guard, Sephiroth enters the Mako reactor containing a large number of pods occupied by living organisms being subjected to an entity known as Jenova. Dumbstruck by the realization that the name coincided with his mother's, Sephiroth for the first time begins to wonder who he really is. Driven by this troubling revelation, he decides to make an investigation of his own into his past history by entering Shinra Mansion and pouring over the volumes of notes and diaries left behind by Professor Gast in the genius scientist's secret library and laboratory down in the basement depths.
From Gast's notes, a project was set up to research the powers of a legendary race known as the Cetra, or Ancients. This superior race was known to be associated with the Promised Land, a place where one can find supreme happiness and where it is believed that it is brimming with raw Mako energy that can be tapped for immediate use. Shinra, the maker of infamous Mako reactors, was froathing at the mouth to get its hands on this pure source of energy. 2000 years ago, a lifeform was found that was believed to be of Cetra descent and was named Jenova, and thus Shinra's Jenova Project took life. Professor Gast was appointed chief scientist to head the project.
Shinra, seeking to replicate the powers of the Cetra, commissioned Gast to produce an elite
fighting corp exhibiting powers of the Cetra. Thus specimens were placed in the pods and infused
and subjected to intense levels of Mako radiation energy provided by Jenova's biological cells.
These specimens, once matured would later become the basic units of Shinra's much revered
SOLDIER regime. Sephiroth was Gast's prize creation and ultimate success story from the Jenova
Project. Sickened by the discovery that he himself was an artificial production resulting from a
demented scientific project, Sephiroth went into isolation within Gast's library and would later
become insane and engulf Nibelheim in flames. Entering the Mako reactor, he would confront his
mother, Jenova, that was placed inside the reactor to provide sustenance to her incubating
children in the pods. Zack, trying to halt Sephiroth's path of destruction, was injured during
combat against his former leader. But it was Cloud who struck the fatal blow and fell the mighty
Sephiroth. Shinra, flexing its political prowess, managed to underscore the Nibelheim incident
and initiated a cover up, dismissing the possiblity that anything untoward occurred. Seeking to
improve on security measures, Jenova was shipped back to Shinra headquarters where Hojo, Gast's
protog� continued monitoring the Cetra's status.
While his physical body was damaged, Sephiroth's being was not to be so easily destroyed. Armed
with the power of Jenova, he would bide his time and later manifest himself. Reclaiming his
mother's body, Sephiroth would thus begin his rage against the world. Standing in his way was
Aeris Gainsborough, the daughter of Professor Gast and Ifalna, a true descendent of the Cetra.
Aeris, able to communicate with the Planet, discovered a way to thwart Sephiroth's true plans.
Entering the Ancient City, Aeris would meet her death at the hands of Sephiroth, but in her
death was a way to save the world, for Sephiroth's motive had nothing to do with the Promised
Land, but rather to obtain the Black Materia and summon Meteor to destory the land. If Meteor
struck, Planet would gather all its Lifestream energy to heal the wound that would have been
caused by the collision. Sephiroth would draw on the entire Planet's energy by appearing at the
source of impact, where a vortex of intense Lifestream energy would gather.
Cloud and company, after retrieving the Black materia back for the second time, approach the Mako crater located in the northern hemisphere, where Sephiroth's physical body is stationed. Cloud, unbeknownst to himself, was himself a creation of Hojo's, injected with Jenvoa cells. His inner being was wrought with turmoil between his true self and the incessant urge to obey Jenova's calling. Cloud loses the mental battle and thus hands the Black materia over to Sephiroth, who then summons Meteor. But in the end, it was Cloud who would vanquish Sephiroth once and for all after discovering his own true identity with the help of Tifa Lockheart.
Although Sephiroth's actions and manipulations were vile acts, he himself was propelled by a devastating truth. He was living an ultimate lie and never did discover his true past. Even when he discovered Gast's notes, Sephiroth was living a falsified truth. Jenova was not a Cetra, but an alien being that the Cetra referred to as the original "crisis from the sky". It was Jenova who infected the Cetra race with a deadly virus which virtually wiped out the entire race, but not before the Cetra were able to band together and confine Jenova in the stratum. But Jenova was later found trapped within the ground, excavated, and mistakenly identified as a Cetra. Jenova was not Sephiroth's mother, nor did Gast "produce" him. Rather, Sephiroth was the bloodchild of Hojo and Lucrecia, Vincent's secret love. Hojo, in his demented quest to find scientific proof to his Reunion Theory, gave up his own son while he was still in his mother's womb to Gast's Jenova project.
One could say that both Cloud and Sephiroth were, in a sense, children of Jenova, brothers bound by a common thread to serve their parent's bidding. While Cloud managed to find himself, Sephiroth lost his way and sought vengeance upon the world. To a certain degree, one can't help but feel sorrow for this character. Sephiroth was a normal child, but was forced to live another man's dream. Living a life not of his own choosing and given the powers of an unnatural evil that would later drive him insane, Sephiroth is the ultimate tragic character.