Since Douketsu has the Ashura Gouenmaru, and all the Kenbumajin's swords have their name implemented into the name of the sword, that means Douketsu might not be the rightful owner. My theory is that Douketsu was envious that he wasn't one of the Genjuu picked by the Gen-ou to become a Kenbumajin, so he decided to steal the Ashura Gouenmaru from the true owner of the sword, which transformed Douketsu into a Kenbumajin and the owner back into a Genjuu. I also theorize that the rightful owner of the Ashura Gouenmaru is Ashura, hence his name being in the name of the sword.