For some reason I see this being having aspects of being both organic and geometric in some ways. And at times being perfectly symmetrical, mostly symmetrical, or completely asymmetrical depending on his mood and tactic taken in his dealings with mortals. And that being done whether he is visible or invisible. Maybe to emphasize his fluctuating hold on the mortal he is influencing.
His presence would definitely be felt more even by ordinary humans if his sway on an individual has pushed them to insanity. It would give them this strange and frightening presence beyond their wild eyes or murderous intent. He can use that to paralyze victims, making them already seemed trapped or like they cannot react or breathe... facing their own demise.