Miharu doesn’t really care about any of that, having a very apathetic personality, and he certainly doesn’t want to be king of the ninja world. The story follows him as he interacts with multiple different ninja clans, each of whom wants Miharu and his abilities for their own purposes. Miharu is left to select between them, in a role he doesn’t care about and doesn’t want. From fairly early on in the series, Miharu has only one goal, to save the life of Yoite, who actually starts out as an enemy from a different ninja clan.
Yoite also has a special talent, a secret ninja killing art which he can use to inflict serious harm on others in exchange for his own life force. So he’s doing his ninja work and slowly killing himself in the process. He tries to hold Miharu’s friends hostage against Miharu granting his wish – which is to cease to exist. Miharu originally agrees with this but comes to desire to save Yoite rather than kill him. Plus there’s no need for Miharu to end his existence as he’s effectively doing that himself by using his power. So Miharu sets out to save him.
Miharu’s personality is weird to me. He’s mostly apathetic and switches sides readily, even against the advice and wishes of the three people trying with all their might to protect and care for him. Those three are his teacher Kumohira Tobari, and two class mates, Aizawa Koichi and Shimizu Raimei. Perhaps it’s explainable since he had no choice in any of it. His mother disappeared when he was very young, and all everyone ever cared about him was his possession of the Scroll with its powerful abilities and his position as Nabari no Oh. It’s probably not surprising that he doesn’t care for any of them in turn until he meets Yoite.
There are several back stories going on here about people’s pasts, including Raimei’s feud with her brother Shimizi Raikou, who seems to have killed off the rest of the Shimizu clan and joined another ninja band, the Grey Wolves. The Grey Wolves is the clan Yoite is with and is also one on the many clans battling for control of Miharu and his ability. During the series various ninja clans begin collecting together all the various secret ninja arts in an effort to either control Miharu or find a way of removing the Scroll from him and having it for themselves. Once Miharu finds out that the Scroll’s primary power is to gift its controller with whatever they most desire, he wants to use it to save Yoite’s life (what he desires most). From that point Miharu sides with whoever is most likely to help him.
The story here is interesting enough and the animation is good enough to keep me watching the entire series. But I really disliked the ending because Yoite doesn’t survive the series. After everything is said and done, all the battles and struggles over and everyone’s basically at peace, Yoite dissolves into dust. I really hate that in an ending. Besides that though, the series was pretty good.