I think I have a theory that not a lot of others seem to be talking about. I find this weird because my theory is more or less told to us in a speech bubble. Anyways my theory is this; Fu and Hii are BOTH vital instruments.
Firstly to clarify, this theory relies on the fact that fu does not have personality disorder in the realistic sense. Instead whenever Fu’s doll became a vital instrument, it became alive as a companion to give orders to Fu as a way for him to absolve himself of responsibility in the same way a persons views or lifestyle effects their instruments power.
Now my theory comes in. I believe that Hii, seemingly having a soul of his own would be able to create anima energy in something else, almost like a person. I also think, however, that Hii, being an object, would operate under different conditions than a human, and would not be able to turn something inanimate into a vital instrument, but would instead only be able to imbue LIVING THINGS with anima. And naturally that being would be Fu. Really what I mean to say is, that line about “it’s like his very flesh is a vital instrument” was more than just hype moments and aura.
This would also likely mean that they exponentially grow stronger every time they switch, as they would be constantly giving the other energy.
Fu and Hii da Goats