Birth by Sleep The True FINAl boss is here!!

Not technically.
Sora used the Heart Unlocker to release their hearts.
Normally, the hearts would make a Heartless, but because she's a Princess of Heart, it didn't, it just returned to her.
Princesses of Heart are special cases, they're the exception, not the rule.
But I guess if someone finds a way to do something similar, they might be able to make a Nobody without a Heartless