Prince Charon said:
Also, I think I need to look up Roa, because Shiakou's description is odd.
EDIT: As far as I can tell, Roa, Ciel, and SHIKI all have separate souls, though Roa's was altered long ago.? That is rather different from Nanya and Shiki, who seem to share the same soul.
Really? Far as I can tell, it's the same soul (keep in mind that Nasuverse souls can be copied, split, and stored as information).
Roa was reincarnated for the seventeenth time as Elesia aka Ciel. In other words, Roa became Ciel, and Ciel/Roa was one being from the time she was born. For some reason, Arcueid was able to "kill" the Roa personality without killing the Ciel personality or the body they shared. (My theory is that the soul was split, though that's only a pet theory and nothing more.) However, both Ciel and Roa are still considered to be the same being; this is the entire reason why Ciel could not die so long as Roa was alive as they were literally regarded by the world as the same being.
Roa was then reincarnated for the eighteenth time as SHIKI Tohno.
In both cases Roa/reincarnation was one being from the moment of birth. The reason for the conflicting personalities is that the reincarnated being has some time to experience the world for his/herself
before the Roa personality begins taking over. Note that the possession does not happen all at once, isn't complete, and isn't always successful; Roa/SHIKI could not perform magic despite being one of the greatest mages to live because he doesn't remember how to
yet by the time of Tsukihime (the manga Roa/SHIKI did not remember how to use magic during his first fight but remembered how to use magic by his second fight with Shiki.)
In short, Roa's reincarnations are more akin to an amnesiac gradually regaining his memories than being possessed by an outside force. The amnesiac might have a totally different personality from his original or previous one, and might even be horrified by what he remembers from his past. The amnesiac can also, with enough willpower, decide to change his life from how he used to live, hence SHIKI basically kicking Roa's personality to the curb in the Far Side Routes.
BTW, all beings are reincarnated in the Nasuverse. In most cases the soul will be destroyed then remade anew (kinda like Fullmetal Alchemist's Alchemy), but Roa made it so that his soul was relatively intact through each reincarnation and was conscious despite being dead. However, Roa's soul was still losing coherence with each reincarnation. It's possible that, with Arcueid killing him over and over again, there might have been eventually a reincarnation who only barely remembered his life as Roa and thus lived a normal, mundane life.