Do you have any Kai and Hiyori headcanons you've been thinking about that may not necessarily fit into the fic you are writing?
Wow, thank you so much for asking this! I’m gonna ramble a bit about how their story plays out in my head. Hopefully, it will make sense in the end. Sorry, this is long.
Alright, so after watching Kai’s story, I’ve got the impression that Asunaro has some cult-like qualities. Like they’ve got science, but there’s also an irrational religious element to their hierarchy. Spiritual leaders can do almost anything, group mentality keeps everyone in check. So I would guess that the Hiyori clan is Asunaro royalty, while the Satous are from a more humble assassin class. These families were never equals, but the relationship between them was polite and civil in the beginning. Momdori would pay Gashu regular visits, and sometimes little Sou would tag along. Whenever that happened, Gashu would bestow Kai with the utmost important task of entertaining the young guest. Even at the age of 8-9, Kai was an extraordinarily obedient, calm and patient child. Unlike other Asunaro kids, he was able to handle Sou’s (~6-7 years old) violent tendencies and intrusive curiosity, considering this challenge to be a part of his assassin training. He introduced Sou to different games, dodged an occasional knife attack aimed at his back, even stopped the younger boy from chopping off a finger or from setting himself on fire.
Sou never caught on that Kai was mostly being nice to him to win father's approval. He adored the older boy to the point of drawing the two of them holding hands, getting married, dissecting people together. Occasionally, Momdori would find those pictures, and her eyebrows would furrow in parental concern. But mostly she’d tell herself that as long as she followed the will of the organization, God would guide her son.
The play dates soured after Sei became part of the Satou family. Sei had zero patience for Sou's antics. And since Sei was older and stronger, their very first game ended with hurt feelings and knocked out milk teeth. Subsequently, Sei perished during the Assassin Game. Gashu's mind started to crumble under pressure. And Kai was faced with the reality of who the Hiyoris actually were and just how much power they wielded.
So when Sou appeared at Kai’s door with a smug smile, hoping to reconnect with his playmate, things did not go well. Kai summoned all the icy sternness he had inherited from Gashu, going so far as to let Sou know that he never liked him to begin with. Hearing this sent the younger boy running home in tears.
This is the part I’m not sure about. Kai and Sou could have just cut contact, which would have lead to misunderstandings and hatred in the future (1). Alternatively, their relationship could have become more detached and formal (2). I started off writing (1), but (2) would be more realistic, probably, since the two of them likely had classes and other activities together with the rest of Asunaro kids. So let's go with (2).
During the following years, Kai likely got to witness Sou's character go from troubling to monstrous. At school, Sou would scheme against older and more popular kids and use his younger, weaker classmates mercilessly for his own benefit. The influence of Momdori on Sou's character was probably huge during this period (we know from her convos with Gashu that she was deceitful and manipulative). Finally, if we’re to believe that Kai started his assassin training in elementary school, then it is likely that Sou was on his way to become an Asunaro businessman by junior high.
Now Kai had lots of pain to deal with during his teenage years and early adulthood. His brother was dead. His father was becoming mentally frail and vulnerable. He was starting to question the system he’s been raised to honor. But perhaps he did try to reach out to Sou at some point, in hopes of stopping the younger man from spiraling. Hopefully, after many years apart, they’ve managed to regain some of the shattered trust. Nevertheless, for Sou, the process of “giving up humanity” was already under way, and he would not turn back. Somewhere a contract has already been signed, the date has been set, and the will of the organization was not to be questioned. Maybe, on the night before the operation Sou confided in Kai that he was scared. Maybe, Kai urged Sou to reconsider his decision. Still, the next day Sou was decapitated, and the doll/AI head was attached to his human body. I imagine it was presented as a sacred ritual, a rebirth of sorts, with some high-ranking Asunaro members present at the scene. Kai was too low-rank to attend. He spent that day cooking for his dad and playing board games with himself.
Ok, I’ll stop here. These are my rough thoughts rn. There’s probably lots of inconsistencies, and I might change my mind on some details later on, haha. But I do like the idea of them sharing some important moments, even though they wouldn't refer to each other as friends.