The meaning of this particular character of "Ku" is a subject of some debate. The character for "Ku" is 空, which can mean "empty", "sky", "void", "vacant", or "vacuum". However, most agree that it probably means Void, as the character points to that, there is already a katana WS named for the sky (Blade: Ten, "Ten" means "sky", "the heavens", "heaven"), and the animation for the WS itself involves the user blinking in and out of sight.