Anti-Aqua is a corrupted version of Aqua. Her hair is a bleached blue, her eyes are yellow, and her once-white dress has darkened and become tattered. Her arms and legs have also begun to darken, and her hands are now black with red fingertips. In battle, she wields the Star Cluster after Mickey drops it during a fight with a Demon Tower.
When she first reveals herself, Anti-Aqua initially appears as a black silhouette wreathed in a violet aura of darkness.
When Ansem, Seeker of Darkness arrives at the Dark Margin to take Ansem the Wise, Aqua tries to fight Ansem, but his guardian subdues her, imbues her with darkness, and throws her into the seas of the Dark Margin, transforming her into Anti-Aqua.
Anti-Aqua later confronts King Mickey and Riku when they return to the Dark Margin to find her. She has her Demon Tower restrain Mickey and takes his Keyblade, explaining the pain she has been through over the years she spent trapped in the Realm of Darkness. She and the Demon Tower then fight Riku, but Riku destroys the Demon Tower and frees Mickey with Sora's help. Sora battles Anti-Aqua and successfully manages to free her from the darkness.
Anti-Aqua behaves nearly identically to the phantom Aqua from Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-. She begins the fight by activating Spellweaver, and continues to fight in this mode. She usually attacks in tandem with doppelgängers, often firing homing orbs at Sora and performing spiraling attacks. All of these attacks can be guarded. She often teleports across the arena, and sometimes teleports before Sora can attack. When her retaliation point is reached, she often counters with an unblockable lunge. Occasionally, Anti-Aqua will wander across the arena with several doppelgängers, explaining her pain to Sora. While she and the doppelgängers will disappear after a short time, Sora can use Thunder to instantly expose the real Anti-Aqua.
As her health dwindles, Anti-Aqua gains access to two ultimate attacks. The first, in which she teleports in the air before firing Blizzard shards, can be guarded or evaded. The second is the Spellweaver finisher, where she and three of her doppelgängers perform a spinning attack. To escape the attack, Sora can either dodge out of the spiral, or block with a Formchange that blocks with barriers.
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