Both Kefka and Sephiroth had extra information 1994 and 1997. You just didn't know. Kefka is mentioned in the Ultimania Omega of Final Fantasy VI.

So too bad for people who don't understand Square doesn't write everything after the stories or people who just don't read the informations you get properly/like the should be understood.

Final Fantasy = Square. With all of its informations. And nothing else.