Except being an aloof sadist doesn't fit with Sephiroth in FF7 to me. In FF7 he's as much a schemer as the rest. He has his plans, and he very much does not enjoy having them left in the dust.
I dunno, he was just a joke, running around with a flower while the other villains did all the actual villainy.
And I have to disagree with you about FF6. FF6 characters have a good amount of personality, assuming we're talking about the top six/seven of them. Terra, Celes, Locke, Cyan, Sabin, Edgar and Setzer all get as much character development as the casts in 7, 8, or 9.
Sephiroth is totally a sadist in FF7...you may want to replay it. He's constantly laughing and sneering while getting Cloud to do his plan for him while he doesn't have to move a finger.
The development of FF6 is mostly imagined in the player's mind. The characters actually do not interact as much as people think. Most people, for example, think of Cyan and Sabin as war buddies, which was intentional from the developers.