Kinger sits in the pillow fort because it's dark and he can actually be somewhat sane, even if that sanity disappears the moment different levels of light enter the picture.
I feel like pomni and him could explore this by testing the levels and then lead to Pomni dragging Kinger to different spots on adventures that have the level of darkness he needs, leading to the others to be suspicious of what they're doing and eventually everyone knows except Caine.