Sephiroth is the embodiment of Cloud's dark side. He is voiced by Lance Bass in Kingdom Hearts. Toshiyuki Morikawa voices Sephiroth in the Japanese version of Kingdom Hearts II, and by George Newbern in the English version. He is regarded as one of the most powerful of all villains in the Kingdom Hearts series (though he is only an option to duel in KH1 and is concerned only with Cloud in KH2), and Sora has to be at a relatively high level in order to survive Sephiroth's attacks.
He, along with Cloud, Tifa, and Cid, comes orginally from the Square game Final Fantasy VII. It has been implied by some fans of Kingdom Hearts in KH2 along with several words from the angel himself that Sephiroth might be some dark version of Cloud. This subplot so far hasn't been touched though it might be shown in future games
Journal Entry
Kingdom Hearts
"Final Fantasy VII"
Once known as the greatest of swordsmen, his whereabouts have been unknown for some time. He challenged Sora to a one-on-one duel. No one knows why he has suddenly returned. Apparently, he and Cloud have crossed paths before.
Kingdom Hearts II
"Final Fantasy VII"
An unsurpassed swordsman who was once revered as a hero. For a long time Sephiroth's whereabouts were unknown, and no one can say for sure why he has returned. He seems to have some connection to Cloud's past.
First meeting
During the Platinum Match, at Olympus Coliseum, Sephiroth appears and fights Sora. He is a secret boss available in all but the regular Japanese version. After being defeated by Sora, Cloud finally meets Sephiroth. Sephiroth implores Cloud to turn to the Darkness, which Cloud refuses. The two engage in a high-speed battle in the air, the outcome of which is unknown.
Second meeting
Right before the invasion of Hollow Bastion, Cloud warns Sora and company about Sephiroth, and his ways of the darkness. Later, as Sora exits the castle to go help defend against the invasion, Sephiroth appears. He asks about Cloud's whereabouts, but Sora doesn't answer, though he and his friends reveal that Cloud realizes that Sephiroth is the manifestation of Cloud's darkness. Seeing that Cloud is becoming enlightened and desires to fight him, Sephiroth takes his leave.
During the battle, Cloud is surrounded by Heartless, but Sephiroth intervenes, defeating them all in one swipe. Cloud intends to defeat his nemesis to escape the darkness, but Sephiroth only replies that Cloud will never be able to do so, and will always cower in the light. Cloud vehemently denies his dark half's claims, but Sephiroth is undeterred, and leaves just as Tifa arrives.
After the Master Control Program is defeated, Sora encounters Sephiroth at the Dark Depths. The villian again asks about Cloud, but Sora and his companions don't reveal anything. He then inquires about Sora, Donald, and Goofy's identities, but again, they refuse to answer. Sephiroth then notices Sora's Keyblade, and attacks, intending to defeat Sora so that the Keyblade might come to him. However, Sora appears to overcome him, but Sephiroth justs seems to shrug off the battle (he appears unhurt, and not even tired, Sephiroth complements Sora on his skill with the Keyblade but says that Cloud is the only that can "eliminate him", Sephiroth then asks Sora to find Cloud so that they can settle their feud).
Cloud arrives later, and a battle begins, with Sephiroth desiring to draw Cloud into the darkness. However, Tifa protects Cloud, saying that Cloud does have light in him, and that, even if he doesn't, then he just needs to be surrounded by light. Nevertheless, Sephiroth and Cloud start off for another battle, but disappear in a brilliant flash of light. Their location and status are unknown.
In Battle
In all incarnations, Sephiroth is portrayed as a master swordsman, wielding a long blade called Masamune. In the Kingdom Hearts universe, Sephiroth has a large repertoire of techniques that reflects some of abilities he used in Final Fantasy VII. He can move about the battle ground with impressive speed, even fly, and occasionally teleports. He can also activate his attack "Stigma" which summons gigantic pillars of flames that can suck matter in, he can send out orbs of darkness called "Shadow Flares", and his most powerful attack "Super Nova" where he calls down meteors that spin around him and then explode. His most noticable technique is Descend Heartless Angel, an attack that drains an opponent of all their magic, leaving the player with 1HP and no MP, if he is not interrupted in time. He can even use Cloud's attack "Omnislash" where he flies around the arena slashing wildly.
In Kingdom Hearts, Sephiroth is one of the hardest bosses, rivaling Kurt Zisa and Phantom, however, in Kingdom Hearts Final Mix, Xemnas (known at the time as the Unknown) also proved to be difficult. In Kingdom Hearts II, he is also one of the hardest bosses, the difficulty of which is rivaled by the challenge of the Hades Paradox Cup and Xemnas again. Kingdom Hearts II: Final Mix+ also has the Organization XIII battles, along with Terra and Roxas, fights that greatly dilute Sephiroth's notorious difficulty.