I got this feeling from Tek, where I think that the doctor or the boss said something along the lines of "The younger they are, the more powerful they can become," which popped into my mind an over-sized hammer or axe for her to wield.
Is that a shotgun slug attachment to those magnums by the way? I'm sold on those then. The tachi is a cool idea for a sword, though they're normally used as a back-up weapon, or dual-wielding. For short swords though, I've always preferred the Russian Kindjal:
Or the Falcata:
Which is my personal favorite because it looks a lot like an umbrella when it's sheathed. The only problem with the falcata is the fact that it's weighted for chopping and that's it. Stabbing, slicing, and cutting are all out of the question because the weighting won't allow it. You risk breaking the sword doing it any other way than chopping.
But, those two probably wouldn't fit with the theme of Japan, so maybe not. I guess that's up to you.
I just found what those magnums reminded me of. Bayonetta's guns.