The relationship Juice Box uses to describe Tagg's fusion with Louis's heart and lungs.
Asked whether she can pin down where the presence sits inside Louis, Juice Box says it is very, very strange: she can sense the lungs and heart, and she can feel Epitome Tagg there. They are symbiotes, in a way.
The classification is what makes the problem unsolvable by force. The chest could be ripped open and the organs taken, but no healing potion would fix it and Louis would die instantly — and the moment anyone moved in, the presence would react.
The medical framing of Tagg's occupation of Louis's organs, ruling out brute-force extraction.
Also appears in books you've already read — spoiler-safe to their endings: