Sometimes when you use drive, Sora takes the form of a Heartless. Though
it gives you some advantage on ordinary battles, it's completely useless
during boss fights. Since neither items nor magics are usable. It only
activates once you use a level 2 drive form.

Anti- Combine with both allies for a faster but weaker form. Wield no
Keyblade. Occurs randomly as you use other forms. Empties out drive gauge if
you revert or leave area. As you use forms you accumulate unseen "Anti-
points", here's how they work:

Change to Valor, Wisdom, Master Form: 1 point each use
Change to Anti Form: -4 points
Change to Final Form: -10 points

0-4 Anti points yields a 0% chance
5-10 Anti points yields a 10% chance
10 + Anti points yields a 25 % chance