My reason for thinking this is, there will be three forms of the little (big) dude:
Overworld - Weak. Loses limbs and fixes them. Does not fight.
Caves - Medium. Fights relatively well
Atrium (soon) - Hard. Said to be stronger than Caves one and should use more of it's animations.
This shows, the closer to the Atrium, the stronger he is.
This would also mean the Atrium is the center of all Nightmare Fuel, which seems to be what truly sustains him.
How does this apply to Charlie?
Charlie on the Overworld (and Caves + Ruins!) usually is just... a spirit. Shadow. Attacking in the darkness, can't be seen.
However, in A New Reign she gains a physical Shadow Form. How does this work? Well, potentially she's even closer to the true source of the Nightmare Fuel than even the Caves.
Perhaps her Throne Room is on the other side of the Atrium Gate, theoretically the true source of Nightmare Fuel.
This Nightmare Fuel would sustain her Shadow Form, and allow her to take the throne (as she did) - What we don't know yet is how she became more... human(?) from being on the throne.
Perhaps one way we could end this is to simply kill the true source of all that Nightmare Fuel.
Who knows.
Just a theory;)