I know what you are talking about (you have found them in FF3usME? ), but most of them are playable by human or you see them move around somehow.

In FF3usME, the character list goes like this:
-All the 14 main character

-Leo, Banon, ????? (ghost), the 12 Moogles that help you in the begining (their names all start with a K exept Mog), Maduin, ????? (Terra at the begining), Wedge and Vicks are all the character you control but somehow get lost in the game

-7 empty slots

-5 Kefka (equipped) and 2 Kefka (nothing on them).

-16 empty slot but with the name set. The names have no relation with anything in the game so I think they are somewhat programmers of the game or something like that.

With that said, I don't think there were character intended to make their appearance but squaresoft decide not to give them the chance to be seen.