Sephiroth

Majority of information taken from Kingdom Hearts Wiki-Sephiroth.

Sephiroth is a villainous character and arch-nemesis of Cloud from the Final Fantasy series who appears in the Kingdom Hearts series.

Kingdom Hearts Birth by Sleep
Although Sephiroth does not actually appear in Kingdom Hearts Birth by Sleep, during the credits a black feather falls before Zack at the Olympus Coliseum, hinting the presence of Sephiroth. Also, Zack mentions to Terra that he aspires to become a hero like "another"; Nomura has confirmed that he was indeed referring to Sephiroth.

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 Kingdom Hearts Final Mix. In the original Kingdom Hearts, it is never shown that Sephiroth and Cloud ever meet, or that Cloud is even aware Sephiroth is a challenger in the Olympus 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 in Hollow Bastion. The villain again asks about Cloud, but Sora and his companions do not 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. Even though Donald and Goofy are with Sora when you confront Sephiroth, they do not fight against him with Sora since Sephiroth only wishes to battle with the Keyblade master. 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", demonstrates that in his battle with Sora he wished to test his power, and see the Keyblade in action.