Anti Form is a Drive Form which appears in Kingdom Hearts II and Kingdom Hearts II Final Mix. It uses both party members, and consumes all of Sora's Drive Bars. It represents the darkness that still dwells within Sora's heart, and is obtained along with Sora's new clothes at Mysterious Tower.
Unlike other Drive Forms, Anti Form cannot be purposefully activated. Instead, it will be randomly activated when Sora attempts to enter Valor, Wisdom, or Master Form, based on a hidden "Anti Point" counter. This counter is increased by 1 every time Valor, Wisdom, Master or Limit Form is activated, but is reduced by 10 when Final Form is used. Gameplay-wise, the Anti Form exists as a consequence of abusing the Drive system, and is meant to be "strong but troublesome".
However, the counter system is ignored while the Two Become One Keyblade is equipped, as its Light & Dark ability will make it so only Anti Form or Final Form activate when Form is used. Anti Form starts at having a 75% chance of being the activated form when Form is used in this state, but goes down to 50% and then 0%, until Final Form is obtained.
Anti Form can only be manually deactivated outside of battle, though its Drive Gauge caps out at 6, and obtaining Drive Orbs will actually cause the gauge to be consumed faster. In addition, though Drive Forms normally persist through cutscenes and Cup seeds, Anti Form will instead revert back to Standard Form. Anti Form is also unique among Forms in that while it possesses the "Anti-Glide" Growth ability, it cannot be leveled and is thus inaccessible in Standard Form. Like Valor Form, Anti Form cannot use magic. It is also not possible to use items. To top it off, Sora cannot pick up HP Balls while in Anti Form, meaning he is completely unable to heal in any way and he also can't gain experience. [Source: wikia]
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