Concept/Headcanon; this isn't based on photos/panels but just a feeling on how we remember their personalities and how their trauma would influence their behavior and feelings on things in their space.
Kaneki (Kid, Human, Kuro) under-decorating because he was so used to never having things to decorate with, that once he did, it freaked out and overwhelmed him enough that he never tried it again. Until; Hide helps him with it, teaching him how to balance it just enough so it's not overwhelming and not aching like he is still an unloved child.
Kaneki (Shiro, Centipede) helping Hinami decorate her room, making sure she knows she is not alone, and never will be if they can be there. No one will feel the way they did as a small child, as long as they can help.
Haise (Haise, Sasako) over-decorating as a coping mechanism because something always feels wrong and decorating stops the "wrong" and "aching" feelings from the empty walls. Like something is missing somehow, but with them over-decorated it reminded them of Hide that they're not alone. (mostly their room)
Yamii (Makkuroneki/Reaperneki/Akaneki) is so under-decorated that it looks like a cell, that can't be comfortable. Surely? No, why should they be comfortable? They don't deserve it. After what they did? They never will. They will never feel like they do.
King let's Hide help, feeling better if they're not alone while doing it, Hide helps with the balance of the room. Not lonely, not suffocating. Helping find fabrics that don't aggravate his rc scales/regenerated parts after Dragon/Ryuu, quiet machines that don't make so much overstimulating noise they start crying and can't explain why, Hide just helps.
