The Void was a sentient thing? It could be counted as a villain?

Anyway, yes, the "one controlling everything" is the main villain. Edea got more screentime than Ultamecia. So did Seifer. But Ultamecia is the main badgirl.

4. Ultamecia: Had to be sealed away in space, still managed to possess Edea.
Adel was the one sealed in space.