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I stole this explanation from the /jp/ thread but beware because it has spoilers from future games:
Kairi is a Princess of Heart, so she has no darkness in her heart, so she can't form a Heartless. If her heart is lost, it doesn't become a Heartless. And therefore, she doesn't form a Nobody. In order for a Nobody to be created, a Heartless needs to be created. Instead, her body just lays comatose with no heart.
Kairi's heart was within Sora. When Sora stabbed himself with the Keyblade of Heart, his heart and Kairi's hearts were released. In order for a Nobody to be created, a heart needs to be released from a body and a Heartless needs to form. Sora's heart was released from his body, which turned into a Shadow Heartless. Which also created Sora's Nobody, Roxas. Kairi's heart was released from a body, Sora's body, so a Nobody was created, which is Namine. Since it was Kairi's heart that was released from a body, then it's Kairi's Nobody. Sora's Heartless acted as the Heartless needed to form a Nobody for both Sora's Nobody, Roxas, and Kairi's Nobody, Namine. Roxas is made out of Sora's body and soul, and Namine is made out of Sora's body and soul, even though she's Kairi's Nobody.
And then Kairi purified Sora's Heartless, most likely using powers she has as a Princess of Heart. She purified him and brought him back to himself. So, he was most likely just a walking, talking heart at this point. So, now he's back to himself. He's back to his somebody essentially. And His Nobody, Roxas, still exists as well. Kairi being there to purify Sora's Heartless is what brought him back and allowed him to exist alongside his Nobody. And since Namine, Kairi's Nobody, was born using Sora's body and soul rather than Kairi's body and soul, that allows Kairi to exist alongside her Nobody as well.
He wasn't a Heartless. Heartless are essentially corrupted hearts. Kairi purified Sora's Heartless, making him no longer corrupted. Therefore, he wasn't a Heartless. He was just a heart.
Also, Ansem, Seeker of Darkness isn't just a Heartless, he's a Heartless inhabiting Riku's body.
Apprentice Xehanort's Heartless is the brown Robed Figure. Which is a Pureblood Heartless. The Robed Figure possessed Riku and stole his body so that he can wield a Keyblade, the Keyblade of Heart. He then sort of molded/shaped Riku's body into a body that's more to his likeness, and became the Ansem, Seeker of Darkness we all know and love. The body he has is Riku's, he just changed the appearance of it. This form isn't Xehanort's Heartless, it's Xehanorts Heartless within Riku's body.
