Antiform

AntiForm (also known as Anti-Sora) is Sora's Heartless Drive Form. It happens based on a hidden counter that increases as you use forms (but drastically decreases every time you use Final Form), and has no abilities.

Most believe that this Drive Form exists due to Sora becoming a Heartless, specifically a Shadow. Since his movements in this Form are remniscent of that of Shadows and Neoshadows, this Drive Form is essentially the closest he ever gets to turning into a Heartless once more.

While Sora is extremely fast in this form, he can only attack without the Keyblade, instead with his hands and feet, and can only revert when not in battle. It uses ALL of Sora's remaining Drive Gauge, and takes both party members. It also makes Sora more vulnerable to damage, and his attack power drops, but his combo length is longer than most other combos in the game (Final Form can get in more hits at its higher levels, but it is very fast, and ends quicker than the Anti-Form combo). Anti-form cannot be levelled up, and has no specific abilities, but pressing Square will make him do a Dodge to Forward Lunge attack (Name unknown).

Formula For Triggering Anti-Form

 * Fights in which Anti-Form never occurs: Pete (all fights), Barbossa (Port Royal), Hades, all battles involving outside characters (i.e. Mickey, Hercules, etc.)

Also, Limit Form is the only Drive Form 100% devoid of any chance in transforming into AntiForm, since it doesn't require Sora to fuse with Goofy and/or Donald, which is also a a requirement for AntiForm itself.

Note: Once the scene where Sora battles Roxas (After you fight him in Final Mix+) has occured, as long as the Two Across Keyblade, acquired from winning the battle and which bears the "Light and Dark" ability, there is a 25% chance you will get Final Form instead. If you still get Anti-Form, it increases to 50%, and then 100%.

Trivia

 * Anti-Form looks very similar to Anti-Sora in the first game (Kingdom Hearts), only without a Keyblade.
 * While Anti-Form is normally impossible to activate manually, certain cheat devices can allow you to activate Anti-Form from the Drive Menu.
 * During the fight against Vexen in Kingdom Hearts: ReCom(and his data battle in Kingdom Hearts II: Final Mix+), Vexen can make copies of Sora that are actually Soras in Anti-Form.