Not that he doesn't fit the theme, but he's way more appropriate as a Ni-Oh boss. Hell, we hadn't seen a Manslayer in the game (yet)
The thing about Nioh is that it takes place in the 17th century, hence all the Sengoku era stuff. The Meiji Restoration (with Ryouma, Izou, the Shinsengumi, etc) is from the 19th.
