system
Given the following post, respond as if you're commenting on Reddit:
human
[Final Fantasy and Kingdom Hearts] Why is Cloud the only person who can defeat Sephiroth?

Sephiroth states in Kingdom Hearts II that "But apparently, Cloud is the only one who can eliminate me." He shrugs off a beating that Sora gave him with no apparent physical damage. 

Is Sephiroth that skilled that the only other person of his caliber is Cloud, or is there something else involved? 

And does this also apply for the source material (Final Fantasy?)
gpt
Cloud and Sephiroth in Kingdom Hearts have a somewhat more nuanced connection than they do in Final Fantasy VII.  Cloud often refers to Sephiroth as an embodiment of his own darkness, and he has a demonic wing on one shoulder to parallel Sephiroth's angel wing.  So while we don't know their specific origin, we do know that they're intimately connected and that, maybe, Sephiroth's life is tied to Cloud's in such a way that even a Keyblade can't take him down.

In Final Fantasy, while Sephiroth and Cloud are connected through the Jenova cells that they share, there's nothing else special that binds them together; in fact there are dozens of people throughout the story who also have those cells inside of them, though none are able to survive and fight their influence to the degree that Cloud does (besides Zack).  Anyone could take down Sephiroth, in theory, though Cloud and his friends are the only ones with the power and plan to take down Sephiroth in the end.