In Final Fantasy VII
Sephiroth is the main antagonist in Final Fantasy VII, who first appears after assassinating President Shinra. As revealed over the course of the game, Sephiroth was once the most powerful member of SOLDIER, Shinra's elite military division, who was celebrated as a heroic veteran of the Shinra-Wutai war. After the war, however, Sephiroth was sent on a mission to the village of Nibelheim, where he discovered that he was the product of a biological experiment that combined a human fetus with tissue from the extraterrestrial lifeform Jenova. Learning that Jenova, who he comes to consider his "mother," attempted to take control of the Planet 2000 years previously, Sephiroth decides to follow in her footsteps and become a god who would rule over the Planet. He burns down the entire village and kills many, but is assumed dead after a confrontation with Cloud inside a nearby Mako reactor, although Cloud does not believe it was he who had killed Sephiroth, whose skill and might dwarfs his, and acknowledges Sephiroth most probably had won but spared him for unknown reasons. However, a few years later, Sephiroth appears once again, determined to continue with his mission.
His plan to become a god is based upon his belief that he can merge with the Planet's Lifestream, taking control of it, and thus the Planet itself. In order to do so, he must summon Meteor, a destructive meteorite entity from outer space that can catastrophically damage the Planet. At this point, the Lifestream will flow to attempt to heal the injury, thus allowing Sephiroth to merge with it. Despite appearing multiple times throughout the game, it is revealed that Sephiroth's physical body is actually sealed in the Northern Crater, and that the manifestations seen by Cloud and his allies were people imbued with Jenova cells taking his form, controlled by the wounded Sephiroth in the Planet core. In the game's last battle, Sephiroth takes two forms; Bizarro Sephiroth (リバース・セフィロス, Ribāsu Sefirosu, "Rebirth Sephiroth") and Safer Sephiroth (セーファ・セフィロス, Sēfa Sefirosu, "Sepher Sephiroth"). After his defeat, Sephiroth reappears in Cloud's mind, but is once again defeated.
In Compilation of Final Fantasy VII
He makes several cameo appearances in the Final Fantasy VII prequel, Before Crisis: Final Fantasy VII, in which he supports Shinra in their battle against the eco-terrorist organization AVALANCHE. The incident at Nibelheim is also featured in the game. The OVA Last Order: Final Fantasy VII also depicts the Nibelheim incident. Sephiroth also appears in Advent Children, a CGI film set two years after Final Fantasy VII. In the film, Kadaj, Loz, and Yazoo, the "Remnants" of Sephiroth, try to reincarnate him. Although Kadaj eventually succeeds, Cloud once again defeats Sephiroth, whose body changes back to Kadaj's upon his defeat. Sephiroth is also the focus of the On the Way to a Smile novella "Case of the Lifestream — Black and White". Set after the end of Final Fantasy VII but prior to the events of Advent Children, the story deals with Aerith and Sephiroth's journeys through the Lifestream, and Sephiroth's creation of Geostigma, a disease that infects anyone who came into contact with the tainted Lifestream. He makes a very brief appearance in Dirge of Cerberus: Final Fantasy VII, a game set one year after Advent Children, in which his biological mother, Lucrecia Crescent discusses the experiments which gave birth to him.
He is one of the main characters in the Final Fantasy VII prequel game Crisis Core: Final Fantasy VII, in which he and the protagonist Zack Fair go in the search of two missing SOLDIERs, Genesis Rhapsodos and Angeal Hewley, This game also depicts the Nibelheim incident, where Sephiroth appears as a boss. Executive producer Yoshinori Kitase was pleased with Sephiroth's role in Crisis Core, feeling that he was given a "much more human side."
His first appearance outside Final Fantasy VII was as a selectable character in the fighting game Ehrgeiz. A redesigned Sephiroth also appears in the North American and European versions of Kingdom Hearts as an optional boss character in Olympus Coliseum. Lance Bass voiced Sephiroth in this game, while in subsequent titles he was replaced by George Newbern. In the Japanese re-release of the game, Final Mix, an additional scene was added in which Sephiroth fights Cloud, although the result of the fight is not revealed. Sephiroth was not included in the sequel Kingdom Hearts: Chain of Memories, as director Tetsuya Nomura could not give him a storyline related to Cloud, and he feared negative fan response if Sephiroth did not have a notable role in the story. His third appearance outside Final Fantasy VII is as another optional boss in Kingdom Hearts II, where he is first encountered by the series' protagonist, Sora, and then Cloud, who is pursuing him. When Sephiroth battles Cloud, both of them disappear, with Sora believing that they went somewhere else to continue their fight. Nomura said that in this game, Sephiroth represents Cloud's dark side, in contrast to Tifa Lockhart, who represents his light side. Although Sephiroth does not appear in the prequel Kingdom Hearts Birth by Sleep, he is mentioned as a hero that Zack Fair aspires to be. The staff, however, did not know if they would portray him as a being of darkness as shown in other titles. Sephiroth's fourth outside appearance is in the Itadaki Street games Special and Portable, where he appears as an unlockable playable character. Due to Sephiroth's many appearances as a boss in the Kingdom Hearts series, Square Enix is not sure whether he will return in Kingdom Hearts III as it would be redundant.
Sephiroth was also the representative villain of Final Fantasy VII in Dissidia Final Fantasy. He is featured in his Final Fantasy VII guise, while an alternative outfit features the "Safer Sephiroth" form. His fight against Cloud in the game was based on their fights from Final Fantasy VII and Advent Children. Along with the rest of the Final Fantasy VII figures in Dissidia, Sephiroth appears in the prequel Dissidia 012 Final Fantasy. This game also includes a sightly altered Final Fantasy VII form for Sephiroth, as well as his Kingdom Hearts form. Sephiroth returns as a playable character opposite Cloud in the third entry, Dissidia Final Fantasy NT. He is featured in the rhythm game Theatrhythm Final Fantasy and its sequel, Curtain Call, as an unlockable character, representing Final Fantasy VII. He also appears in the puzzle platformer video game LittleBigPlanet, and its sequel LittleBigPlanet 2 as a character model; Media Molecule's Alex Evans felt "honored" that Sephiroth was allowed to appear in the games. He is referenced in Super Smash Bros. for Nintendo 3DS and Wii U during Cloud's reveal trailer, and in an in-game trophy description.