Originally Posted by
TwilightsCall
When Roa reincarnates, he takes over the body of someone else he deems "worthy," and when they are sufficiently mature his memories/personality reawaken and he "reincarnates." The issue is that Roa is there the whole time, and ostensibly usurps the body from whatever soul it originally would have had. This raises a couple questions: when is a human endowed with a soul? If it is at birth, then one of Roa's hosts never would have had their own soul: Roa's soul would be the only one in the body. If it's before birth, at a time before Roa has transmitted his soul into the baby, then the question becomes: what happens to that soul? Does Roa's soul like, eat it or something? Do they co-exist peacefully (at least until Roa's soul just takes over)? Or is it expelled/suppressed only when Roa takes over?
From what I remember (and it's been a while, so I may not remember very well), there is no latent "human soul" that Roa is fighting against when he takes control of the body. There's just a transitional period where the previous personality is overwritten by Roa's, or at least what's left of it after so many reincarnations. But from the beginning there's still just Roa's soul in the body, not someone else's.
The question for Ciel then becomes, if Roa's soul has been expelled from the body, what happens to her? After all, since Ciel is immortal because "her" soul is Roa's soul and so the world won't let her die before her soul dies, that implies her body just doesn't have a soul of it's own. This raises a lot of questions about how important the soul actually is in the Nasuverse, to the point it makes me think it just can't really be true.