It's either Mizuki's father's sword, or the sword that killed her, I can't remember which.

But the bit with Benitora...that's kinda what I meant...even Benitora has to have special abilities to keep up, but Yukimura's better than Benitora with nothing more than a sword. Sadly, the character type is never allowed to remain as good as everyone else despite no powers, and always has to get a power to be one of the cool kids.