[@Demonic Angel]
	well considering her heart is basically sora's heart, who for some reason is now living without a heart from transforming out of a heartless form. which technically his now basically got the power of a heartless which was combined into his transformation power in #2 (Anti-Sora form that could randomly happen love the transformation). so if Kairi's heart is actualy Sora's then it kind of makes sense, it's just that she had her memory erased (Unlike riku) for some time, and never had to fight the heartless herself so she never could call it before she needed it in #2.