"I Am Sephiroth."
I was the product of ShinRa's "Jenova Project"; injected with Jenova's cells while in the fetal stage by my own so called father, Professor Hojo. I was separated from my "Real Mother Lucrecia" and was raised by ShinRa, who had me believe Jenova was my mother. I became a quiet and strict member of ShinRa's SOLDIERs during the Wutai war.
"Some Infomation About Him From The Game To The Movie"
Five years prior to Final Fantasy VII Sephiroth, Zack and Cloud (a ShinRa MP at the time) were sent to Nibelheim to investigate the old Mako Reactor in the mountains. When he sees the preserving area of Jenova, Sephiroth's world is then turned upside down. From reading Gast's findings on the Jenova Project, Sephiroth learned he was nothing more but a experiment. The horrible truth drove Sephiroth genocidally insane, brutally murdering the 'traitors' of the Planet; the regular humans. He is responsible for the death of Cloud's mother and Tifa's father. He injured Tifa and Zack while he was mortally wounded himself by Cloud. But Sephiroth dived into the Lifestream below the reactor with Jenova's head on him. As far as everyone knew, Sephiroth was dead....
But in truth, Sephiroth drifts through the Lifestream and ends up in the Northern Crater, frozen. However, from integrating some of Jenova, Sephiroth learned that he controlled the "Sephiroth Copies" with his mind, as they possess Jenova's cells in them like Cloud. He also can manipulate the rest of Jenova to take on his appearance, taking over Jenova's body and using it to kill President ShinRa.
Taking Cloud and company on a wild goose chase with his "puppets", Sephiroth had them get the Black Materia which Cloud gave to him at the "Reunion" Sephiroth arranged at he Northern Cave. While Meteor was heading to the Planet, Sephiroth absorbed every remaining piece of Jenova (the copies that were slaughtered and his "puppets") to assume a cocoon-like stage (Bizzaro Sephiroth) and by the time Cloud and gang arrived, Sephiroth emerged as an angelic figure from absorbing some of the Lifestream (Seraph Sephiroth).
Though he was killed, Sephiroth's spirit intended to kill Cloud and take over his body. But Cloud overcame Sephiroth, whose soul was taken by the Lifestream. But by Advent Children Sephiroth's scattered spirit became the three Remanent, who are to find Jenova's head, and once absorb become Sephiroth. Kadaj succeeded and Sephiroth was reborn. In the end, Cloud performs the Omnislash 5.0, the new version of Omnislash. Sephiroth shows his infamous black angel wing at his defeat, whispering his final farewells as he fades back to the Lifestream for good.