Sephiroth is the main antagonist throughout Final Fantasy VII and Advent Children/Complete though he was not always the villain. After being raised within ShinRa as part of the top-secret Jenova project he was the legendary hero of the Wutai war, the best General in SOLDIER; before the truth was discovered about his origins and he went 'insane'. It's complicated in that it could well have been the intention of the Jenova project (once taken over by Hojo i/o Gast) to have turned him into the villain that he became. Rather than his own conviction that he was fulfilling his own destiny by attempting the planetary take-over Jenova had failed to do whilst also taking revenge on those responsible for lying to him and/or stopping Jenova in the first place (aka the entire human race).
It all started with the events just prior to the 'Nibelheim Incident', in which Sephiroth started to look into ShinRa's G-Project following Genesis' defection. This was soon to be followed by Angeal's defection as well and as if it wasn't bad enough to have his best/only friends betray his entire life of being loyal to ShinRa, Sephiroth's world started to unravel around him as he dug deeper into research on the G-Project. He'd even hinted to Zack, that he might be leaving SOLDIER. All this tension came to a head at Nibelheim, where Sephiroth learned of the truth of his own origins and his sanity cracked - hence the Incident. After taking Jenova's power into himself within the Lifestream afterward, Sephiroth had no intention of stopping there. Enlightened with Jenova's power and the vast knowledge absorbed from the Lifestream directly, Sephiroth went about his Reunion and would use the Ultimate Destruction Magic: Meteor to propel him into that God-status with which he could complete his destiny/revenge. Only to be stopped by the same one who had sent him into the Lifestream the first time. His 'failed clone', his 'puppet' - Cloud. Revenge for all this betrayal, added to the revenge to-be-taken against those that have stopped him twice already (especially Cloud) forms a large part of Sephiroth's drive especially in post-OGC canon.
There is further canon back story available on wiki's/derived from FAQ's etc but it's not many of the the events pre-Nibelheim matter very much considering this canon pull-point.
(Refer to Memory weakness) A great bulk of Sephiroth's memories are shattered/missing, though through use of the remnants in Advent Children/Complete - picking up memories of himself from other still-living people within the canon whom the remnants came across, some of them are at least partly recovered. Details such as his appearance, skills and key aspects of his personality had to have been recovered in this way; though it's not ever stated in canon which memories Sephiroth did recover this way but it must have been sufficient for him, coupled with the vast amount of Lifestream he'd managed to gain influence over with Geostigma, to will himself back into being through Kadaj when reunited with the last remaining Jenova cells. This canon pull-point is just after he's been forced into retreating his will back into the Lifestream after being defeated by Cloud in the ensuing fight - though losing the battle might not mean he might lose the war. As long as Cloud continues to remember him, his presence will still be within the Lifestream.
[CANON] Sephiroth || Final Fantasy VII || reserve || 3 of ?
Wiki Link (here)
Additional: OTWTAS Novella: Case of Lifestream - Audiobook courtesy of TLS
Sephiroth is the main antagonist throughout Final Fantasy VII and Advent Children/Complete though he was not always the villain. After being raised within ShinRa as part of the top-secret Jenova project he was the legendary hero of the Wutai war, the best General in SOLDIER; before the truth was discovered about his origins and he went 'insane'. It's complicated in that it could well have been the intention of the Jenova project (once taken over by Hojo i/o Gast) to have turned him into the villain that he became. Rather than his own conviction that he was fulfilling his own destiny by attempting the planetary take-over Jenova had failed to do whilst also taking revenge on those responsible for lying to him and/or stopping Jenova in the first place (aka the entire human race).
It all started with the events just prior to the 'Nibelheim Incident', in which Sephiroth started to look into ShinRa's G-Project following Genesis' defection. This was soon to be followed by Angeal's defection as well and as if it wasn't bad enough to have his best/only friends betray his entire life of being loyal to ShinRa, Sephiroth's world started to unravel around him as he dug deeper into research on the G-Project. He'd even hinted to Zack, that he might be leaving SOLDIER. All this tension came to a head at Nibelheim, where Sephiroth learned of the truth of his own origins and his sanity cracked - hence the Incident. After taking Jenova's power into himself within the Lifestream afterward, Sephiroth had no intention of stopping there. Enlightened with Jenova's power and the vast knowledge absorbed from the Lifestream directly, Sephiroth went about his Reunion and would use the Ultimate Destruction Magic: Meteor to propel him into that God-status with which he could complete his destiny/revenge. Only to be stopped by the same one who had sent him into the Lifestream the first time. His 'failed clone', his 'puppet' - Cloud. Revenge for all this betrayal, added to the revenge to-be-taken against those that have stopped him twice already (especially Cloud) forms a large part of Sephiroth's drive especially in post-OGC canon.
There is further canon back story available on wiki's/derived from FAQ's etc but it's not many of the the events pre-Nibelheim matter very much considering this canon pull-point.
(Refer to Memory weakness) A great bulk of Sephiroth's memories are shattered/missing, though through use of the remnants in Advent Children/Complete - picking up memories of himself from other still-living people within the canon whom the remnants came across, some of them are at least partly recovered. Details such as his appearance, skills and key aspects of his personality had to have been recovered in this way; though it's not ever stated in canon which memories Sephiroth did recover this way but it must have been sufficient for him, coupled with the vast amount of Lifestream he'd managed to gain influence over with Geostigma, to will himself back into being through Kadaj when reunited with the last remaining Jenova cells. This canon pull-point is just after he's been forced into retreating his will back into the Lifestream after being defeated by Cloud in the ensuing fight - though losing the battle might not mean he might lose the war. As long as Cloud continues to remember him, his presence will still be within the Lifestream.