The anti-form is annoying, isn't it?! Tetsuya Nomura described the form as a resonance of Sora's brief time as a Heartless. I thought it was pretty cool at first (it moves so quickly) until I realized it drains my drive gauge and takes double damage. -_-

Once you unlock the Sora/Roxas battle scene, keep transforming into your forms (any one) DURING a battle until you get the final form. Once you've used the final form a few times, your points will be so low that there will be almost no chance of getting the anti-form for the rest of the game =)