Sephiroth

Sephiroth is the embodiment of Cloud's dark side. He is regarded as one of the most powerful 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). It is recommended that Sora be at a relatively high level in order to survive Sephiroth's attacks.

He - along with Cloud, Tifa, Aerith, Yuffie and Cid - come originally from the Square game Final Fantasy VII.

Kingdom Hearts
''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.''

"Final Fantasy VII"

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.

Kingdom Hearts
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 having been defeated by Sora, Cloud finally meets Sephiroth. In silence, 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 (however, this is only in the Final Mix version of KH1. In the original, it is never shown that Sephiroth and Cloud ever meet, or that Cloud is even aware Sephiroth is a challenger in the Coliseum)

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 one 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, and after Tifa's intervention, Cloud glows in light, surprising Sephiroth and letting his guard down. The two 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 their encounter in the first game never happened, since Sora, Donald and Goofy don't know who he 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 quote - "show me your strength" demonstrate that in his battle with Sora he wished to test his power, and see the Keyblade in action.

Although he appears in Hollow Bastion/Radiant Garden, it is not said whether he resides there permanently or not, which can also mean that Radiant Garden may not be his home world.

In Battle
In all incarnations, Sephiroth is portrayed as a master swordsman, wielding a long blade known as the 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 also use a variation of Cloud's Omnislash, where he flies around the arena slashing wildly. In both games, all of his attacks are extremely powerful, being able to easily kill Sora even when he's maxed out. Sephiroth as has extremely high HP in both games. 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.

Magic Stats
In Kingdom Hearts, Sephiroth is immune to almost all forms of magic although 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 Kingdom Hearts II Final Mix, all Limits in Limit Form work well against Sephiroth.

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 Meteor and Flash, are still named for attacks in the series.
 * Some of Sephiroth's moves from the Kingdom Hearts series have been seen in use 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. The Meteor attack from both KHII and Dissidia involve a glow of sorts and the Heartless Angel both shows feather-shedding. In the English version of Dissidia, he is seen using the pop-up attack from Kingdom Hearts II.
 * Additionally, Sephiroth's redesigned appearance which granted him a black wing seems to have inspired Nomura to use the one-winged concept in the Compilation of Final Fantasy VII where characters infused with Jenova cells grow similar wings on their backs as well as Sephiroth sprouting one during Advent Children.
 * Sephiroth's Heartless Angel technique was originally first used by the Final Fantasy VI primary antagonist, Kefka Palazzo. It would later reappear as an attack of his in Crisis Core - Final Fantasy VII.
 * Sephiroth's original English voice actor, Lance Bass of NSYNC fame was replaced by George Newburn in all succeeding appearances due to negative reactions. Newburn originally voiced Mevyn Nooj from Final Fantasy X-2 and outside the franchise, had provided the voiceover for Superman in animated spinoffs since the 90s.