Heartless Sora?
When you change into a drive mode sometimes sora turns into a really powerfull dark shadow for a while. Just wondered why seeing as the game gives nothing away.
"marmite" love me or hate me :p
There's an invisible counter that marks how often you use your drive forms and once the counter is filled you turn into Anti-Sora (aka Dark shadow). The only form that doesn't risk the Anti-form is Final form (The white costume)
The following has some spoilers with my own theory:
The reason behind it is never revealed in canon. I think that the reason is that Sora isn't "whole" in the other forms. He is drawing people that aren't his own heart into him and riskes adding their darkness to his heart. Since Final Form is Roxas and Sora, who are the same person, are truely binding to release thier full potential Anti form can't be reached.