I think the term heartless is misleading. A heartless refers to someone whose heart was consumed by darkness, not someone without a heart. So they should be called 'dark hearts'. Anyway, only Kairi's body was lost in the darkness. So she never became a heartless because her heart was just fine. In fact, as a princess of heart I would think it impossible for her to become a heartless since her heart is pure light.

And I can't believe I'm such a huge dork for knowing all this.