It's because she's a nobody born from Sora's body. She came to be at the exact same time as Roxas. When Sora released Kairi's heart,(and his own) they were born. He was sort of like a medium. A nobody is born when a person with a stong heart becomes a heartless. The conditions were met because Kairi's heart was inside Sora. And HE became a heartless. So technically His and Kairi's stong heart created nobodies. Kairi own body didn't turn into a nobody because her heart never(impossible) left the realm of light. So she was stuck in a comatose state. She didn't become a heartless but she was technically heartless. Namine looks that way because she is a nobody born from Sora's body.