I think it’s more like the “window beings” are separate entities that “take care” of them in a weird way since childhood until hot author guy (forgot his name lol) learned to speak and control them.
THIS
OOOOWWOOOO YOUR THEORY MAKES A LITTLE MORE SENSE BUT THEN WHY THEM IS MY QUESTION. WHAT MADE THESE MIRROR SPIRITS SO ATTRACTED TO THEM TO THE POINT WHERE THEY OBEY/PROTECT THEM
They are providing visual stimulation ლ(´ڡ`ლ) Na' I don't know. Maybe there are more like them and they can only control the ones they kill. Alsa want to know if the adoptive father is actually a good person or not.
OR maybe hot author guy and uke has this “collective imagination” thing going on (I cannot word this properly ugh), imagining these creatures in the mirror to be existent when it’s just hot author guy killing these people.
Another might be that these creatures want the two to become part of them, witnessing their traumatic experience during their early years in life and thus making them easier to lure to the dark side (not that traumatised people are like this, no. I just link this theory to the plot so...)
GUYS WAS MY THEORY CORRECT DOES SIR OFFICER HAVE POWERS AND JUST USED THEM ACCIDENTLY