@Toloran Hmm... The last theory is the straightforward reincarnation theory that I've been running with. (Ryumatsu is just a past live of Princess Ryuu.) Note that there's another hint: That Princess Ryuu was the one who named Usain and Bolt while she was young -- signifying that she already had bits of his knowledge.
So if I had to bet, then I would bet that author is just trying to throw us off the trail at this point, and that it is straight reincarnation.
Update: Oh, thought of an interesting theory (which I've now added to my original wall of text): Maybe it
is different souls, but Ryumatsu was added at Princess birth and just didn't manifest until later. This is a little convoluted, but it fits well -- it explains Usain/Bolt naming and why Princess/Ojiisan are so tightly connected.