Final Fantasy VII
Sephiroth is the main antagonist in Squaresoft's Final Fantasy VII. Sephiroth was the highest ranking member of SOLDIER, and was thought to be dead five years before the events of Final Fantasy VII. At the beginning of the game he is vaguely mentioned in some dialogue, but is later revealed in some flashbacks from the game's protagonist, Cloud. He is the child of Lucrecia and Hojo, and was injected with Jenova cells, which explains his extraordinary power. However, when Cloud asks about his parents, he laughs and says Jenova is his mother, not knowing of his own past.
The first actual appearance of Sephiroth in Final Fantasy VII is in the Shinra Headquarters. Cloud and the rest of the party have been captured and are awaiting execution, but after a night's rest, they awake to see the guards are dead and their cell door is open. Following the trail of blood left by the Shinra guards, they find the President of Shinra dead, with Sephiroth's sword thrust in his back. After seeing this, Cloud vows to kill Sephiroth, and settle the score in Nibelheim. During Cloud's flashback at the Kalm Inn, he, Sephiroth, and Tifa all go to a reactor deep within Mount Nibelheim. There is a brief battle with a Green Dragon (Sephiroth kills it quickly), where Sephiroth's power is shown for the first time.
Later at the reactor, he tells Cloud that the creatures in the tubes are actually people infused with a large amount of Mako, more than that of members of SOLDIER. He sees Jenova in a larger tube and begins questioning his past, and how he was created. He holds up in the basement of the Shinra Mansion in Nibelheim, where he reads about Professor Gast, and many experiments that have been done. He becomes obsessed with finding out how he was born, and finally goes insane. He burns Nibelheim to the ground, killing Cloud's mother in the process, and runs to the reactor in Mount Nibelheim to find Jenova. Cloud chases after him to find that he murdered Tifa's father. Tifa chases after him but is struck down, and is saved by Cloud, just as he promised during their childhood. Cloud rushes in to confront Sephiroth and that's where his story ends.
Cloud doesn't remember why he and Sephiroth are both alive, or what happened. Later in the story, the holes in Cloud's memory are revealed. In Nibelheim another charactor named Zack was the member of SOLDIER with Sephiroth, not Cloud. Cloud was never in SOLDIER and was embarrased to go back to his hometown, thinking Tifa would look down on him for being a normal Shinra soldier. Cloud was still there, however, but masked his appearance. At the reactor Zack actually rushes in to fight Sephiroth and is struck down.
As Sephiroth is leaving the reactor, Cloud confronts him and throws him into the lifestream, but is mortally wounded in the process. Much later in the game, it is revealed that Sephiroth plans to use the Black Materia that he manipulated Cloud into giving him to summon a powerful spell called Meteor. By summoning Meteor, he would gravely wound the planet, causing the Lifestream to rush to where the meteor hit, and giving Sephiroth an opportunity to merge with the planet itself, giving him god-like powers. Aeris tries to stop him by counteracting his Black Materia with her White Materia, and tries to summon Holy. Her plans are cut short as Sephiroth kills her before she can finish casting the spell. Sephiroth then retreats, barricading himself in the Northern Crater, where he continues to summon Meteor. At the climax of the story, Cloud and AVALANCHE proceed into the crater after Shinra destroys the barrier with their Sister Ray. Sephiroth tries to stop them with his multiple forms, but in the end falls to Cloud in a mental battle, after Cloud recovers his memory.
The portrayal of Sephiroth in FF7:CC is vastly different to his presentation in FF7. At the start of the game, Sephiroth is known as the hero of the last great war, with the neighboring country, Wutai. Hailed as the man that stopped the war, he is employed by the Shinra as a member of SOLDIER: 1st Class, an elite group used by Shinra on dangerous missions. He is good friends with Angeal and Genesis, fellow 1st class members. After Genesis and Angeal defect from Soldier, Sephiroth refuses to give chase to his two friends, deferring the mission upon Zack, an up and coming Soldier, despite being 2nd class. Following this, Sephiroth continues to chase down Genesis, especially so after Angeals death at the hands of his former friend, Zack.
The Fall of Sephiroth
During a mission to Nibelheim, Clouds hometown, Sephiroth discovers a secret lab underneath the Shinra Mansion in which he finds out that he was created from the cells of his mother, codename "JENOVA". Up until this point, he was aware that his mothers name was Jenova, but didn't connect the two. Following this, Sephiroth slowly descended into a spiral of madness. He locked himself up underneath Shinra mansion, becoming consumed by finding out his origins, and purpose. After this, Sephiroth burns down Nibelheim in rage, and continues on to the mako reactor in the Nibelheim mountains, in search of his mother. Upon arrival, he finds the corpse of his mother, and strikes down Tifas father, aswell as wounding Zack in the process. After seeing this, Cloud charges towards Sephiroth and impales him with Zacks "Buster Sword", and throws him into darkness. For the purpose of Crisis Core at least, this is the last time Sephiroth is seen. Cloud does not re-encounter him until several years later, in Final Fantasy VII.