#cinna's analysis

the notable thing that makes it so that hakusumi's yorishiro case is different from mitsukou's is that hakubo kind of owns sumire (which has some interesting implications), due to her family literally passing her onto his hands. that's likely the reason why a human can be one's most important posession, because the mystery likely must feel entitled to some part of that person in order to make them their most important posession.

however, a key factor in mitsuba's story is that he feels like he doesn't belong anywhere, much less own anything at all. theoretically then, in order for a true mitsukou yorishiro au, mitsuba would have to feel like he owns a part of kou, or at least stakes some claim over him.

even then, there's been a second case of a human yorishiro, and it's much closer to mtsk's situation than to hakusumi, and that's the yugi twins.

i believe that the difference there is that hanako knows he is a selfish being, and has fully accepted that he's selfish, making selfish actions despite knowing that he shouldn't. it's not far-fetched to think that he feels like he owns a part of tsukasa, or has a right to his own brother, making tsukasa his most important posession even though he isn't even sure of whether that is tsukasa or not, because tsukasa must mean that much to him. it's a line of thinking that mitsuba doesn't allow himself to access.

because still, mitsuba (a supernatural, a cannibal, a "fake") believes that he shouldn't get involved in kou's human affairs, and that cutting kou off will do the best for both kou and him. he hasn't grown into the selfishness that hanako lives with yet, but that only when he does will he be able to say that he lays some sort of claim over kou, and then make him his yorishiro.

extra note: this system also makes it clearer that despite the thing living in tsukasa, it is tsukasa inherently; because hanako's most important posession is tsukasa, not the thing. maybe he did believe that it was his brother, but ignored and rejected the possibility, but the yorishiro system somehow made tsukasa his most important possession? ... some interesting conclusions to be drawn about how the system works, perhaps?