They never truly said that the True Warrior and True Demon were supposed to fight. All they said was that the true warrior would have absolute power and the true demon would have all of those powers, except use them for their own purposes. They never said that they had to be two separate people. Like in my previous post, it seems like the true warrior/demon/"god" could be one person, but it depends on what that person does with that power. I'm starting to believe if this person does exist, then it might be nagi. Without him being the main hero, he's definitely changed since the beginning of the story. The thing about him is that it seems as if he gains the best qualities from his friends and family, which helps him control his dragon. His dragon is definitely powerful, probably the most powerful out of any red feather in the series.
He isn't the annoying "i'll fight anyone and everyone just to get stronger" kid from the beginning. He's fighting with a noble purpose and he knows just how strong he is compared to the more normal characters. With regards to Souichiro/Sohaku merger, Souhaku will have to awaken or else they can't revive Maya. The thing is, Souichiro could possess the ability to break Souhaku's control with his own spirit.