The Uncanny Sword and Tsubaki's older brother. His weapon form is a black katana that can manipulate his wielder's shadow. He became jealous when his sister inherited the majority of the Nakatsukasa family's multiple forms, believing himself to have been more worthy. This jealousy caused him to begin devouring human souls by possessing them through his weapon form, threatening to transform into a kishin. After he battles Black Star and Tsubaki while possessing a boy named Ryoku, Masamune and Tsubaki's souls merge, allowing Tsubaki to defeat and redeem him, absorbing his soul and the power to use his Uncanny Sword mode.