Kotsu no O

is a kettou jouken that belonged to an unnamed Demon and is now in possession of Nox. Kotsu no o manifests as a permanent bone tail that acts as an extension to the body, given that it is a "bone" tail, its use with Demons makes it compatible with their Brimbones. The shape of the tail differs with each user, often appearing to have ribbed segments or a sharpened tip at the end.