Boy ghost attacked with salt: story from chapter 29.
Mom throwing what looks like salt in the room affecting our POV: end of chapter 35
Other Clues:
We are at least somewhat portable; Yuuma wrapped us in a blanket(?) and brought us with him when he was imprisoned in the closet (ch 24)
0ur pov can be tilted and Yuuma can manually adjust it (ch 2)
Yuuma can also cover up or block our pov with other objects: a handkerchief (ch 2,) a notebook (ch 3,5,) and Umikin (ch 14.) The handkerchief in particular completely covered our pov, and we could still see the handkerchief itself.
Our pov can also go completely dark. This happened in ch 15 after a shadowy male figure came in and reached towards us. It also went dark for a bit when we(?) stabbed that lady's hand with a pencil (ch 20,) but
not when the cop fell out the window (ch 37.)
Interestingly our pov didn't change at all when the shadowy figure completely upended the desk (ch 29.) Which suggests that we aren't actually
on the desk.
Yuuma has expressed relief that we're "safe" and that other people in the house didn't "find us" (ch 24, 27,) and also the police officer noticed us this chapter. Which suggests we aren't invisible. On the other hand Yuuma keeps us in a pretty central location of his room and doesn't usually cover us, even when he's out of the house. Which suggests we're either hard to see or not super interesting looking.
I did notice that there were scraps of paper on Yuuma's desk at the start of chapter 16 (right after the shadowy figure made our pov go dark)
I'm going to go out on a limb and guess that we're looking through a talisman or something that Yuuma made out of paper (hence the scraps) and put on the wall behind his desk (hence overturning the desk not working).