It's been so long, what relationship did Shinozaki have with Mahiru again? And what is this talk about compatibility between the caster and repository?
Legally, Mahiru is the one who turned Shinozaki into a human repository 15 years before the start of the story. We have been shown brief flashbacks that he was in a major accident around that time, but has no memory of the specifics.
As for compatibility, it has been reiterated repeatedly throughout the series and showcased with each and every single human repository known in the story.
- Hazakura's desire to be praised is mirrored in her human repository, who recognized this when the magic was cast, and then used that knowledge to completely change her world. She transformed from an expressionless stone-faced child to who she is today.
- The noodle incident.
- Asahina shares her desire for control and reckless taste for the questionably legal with her human repository, who then acknowledged her for who she is without judgment. Perhaps in 200 years she will be better for it.
- Konpeito's absent internal north star has plagued her for seemingly centuries and prevented her from ever having a steady family or community presence, but yet again, this is something shared by her human repository, whose lack of motivation proved that she was not the sole black sheep of the world.
It is said that only one or two people in the world have the proper aptitude to become a magician's human repository, likely for this very reason. The magician and human need to share a fundamental characteristic or feature between each other. Shinozaki's amnesia obscures the true nature of this connection, as well as with whom it was made.
Edit2: I should add that in much the same way, the human repositories above were saved by meeting their kindred spirits and having a mirror to reflect their own inner desires.
Edit:
We also have no clues yet on why his head falls off or for what reason his entire body is a viable opening, which based on this chapter should be connected to his sense of self. Notably, we've also observed:
- Hazakura's human repository opens at the mouth, and their shared wish was to be praised.
- Asahina's human repository opens on one side of her face, and both of them have felt conflicted over the gap between others' perceptions of their intentions and their actions.
- Konpeito's human repository opens at the chest, and they are both "empty" inside with a lifelong struggle to connect with any peers.
It could be because of whatever traumatic incident occurred at the time. It certainly seems to be unraveling quickly now. I wonder if he's been experiencing the full flashback during this chapter's course of events. At the very start of the story, when faced with supernatural threats and challenges, he claimed he wished to have a family and for other things in line with societal norms. Was that just a gag?