Sephiroth

Sephiroth is the embodiment of Cloud's dark side. He is voiced by Lance Bass in Kingdom Hearts, Toshiyuki Morikawa 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 Kingdom Hearts and is concerned only with Cloud in Kingdom Hearts II), and Sora has to be at a relatively high level in order to survive Sephiroth's attacks.

He, along with Cloud, Tifa, Aerith, Yuffie and Cid, comes originally from the Square game Final Fantasy VII. It has been implied by some fans of Kingdom Hearts that in Kingdom Hearts II, along with several words from the angel himself, Sephiroth might be some dark version of Cloud. This subplot so far hasn't been touched though it might be shown in future games.

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.

Kingdom Hearts II
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 villain 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 test Sora's power and that of the Keyblade. Sora appears to overcome him, but Sephiroth just shrugs off the battle. Sephiroth complements Sora on his skill with the Keyblade but says that Cloud is the only that can "eliminate him". Sephiroth then tells 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.

Despite the fact Sephiroth and Sora can battle in Kingdom Hearts, his dialogue with Sora in Kingdom Hearts II implies that there encounter in the first game never happened, since Sora, Donald and Goofy don't know who is, and Sephiroth doesn't seem to know them either. However, Sephiroth says "so that's a Keyblade", indicating that while he may or may not have seen a Keyblade before, he at least knows of them. His battle quotes - "show me your strength" and "what an intriguing power" - demonstrate that in his battle with Sora he wished to test his power, and see the Keyblade in action.

Kingdom Hearts: Birth by Sleep
Although he hasn't been confirmed, in the "Gathering" the Fenrir Keyblade can be seen. This lead to the speculation that Sephiroth would appear in Birth by Sleep, or Tifa, who has the Fenrir Keychain. There have been no characters from the Final Fantasy franchise confirmed for Birth By Sleep yet, and Tetsuya Nomura had stated that their were many keyblades already used as "fillers" in the scene, meaning this speculation could be a red herring.

Magic Stats
In Kingdom Hearts, Sephiroth is immune to almost all forms of magic. The only spell, in fact, that does damage to him is Aero magic at it's highest level (Aeroga). However, magic based attacks (such as Ragnarok) do in fact cause damage

In Kingdom Hearts II, all attack magic has no effect as well. The only magic based moves that will affect him are Magnega, the aftershock of Reflega, and the magic combo of Trinity Limit.

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 noticeable technique is 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. To add to the difficulty of him being a powerful enemy, in both games Sora has to fight him one-on-one without any help.

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, having no true rival, standing out as the games hardest boss. Kingdom Hearts II: Final Mix+ also has the Organization XIII battles, along with Terra and Roxas, fights that greatly dilute Sephiroth's notorious difficulty.

Trivia

 * Almost all of Sephiroth's attacks, such as Super Nova, Heartless Angel and Shadow Flare, are named for attacks he uses in the Final Fantasy series. The few attacks he does not know in the Final Fantasy series, such as Meteo and Flash, are still named for attacks in the series.
 * Some of Sephiroth's moves from the Kingdom Hearts series had been recently translated into the crossover fighting game Dissidia -Final Fantasy as well as Crisis Core, due to the circumstances and limitations concerning the original Final Fantasy VII. Such attacks have made an appearance such as the Reaction attack used by him in the second game as well as manifesting darkness orbs and teleporting.
 * Most likely because of the release of Final Fantasy: Advent Children, Sephiroth's voice actor was changed to George Newburn in Kingdom Hearts II, since he voices Sephiroth in that movie.