Ok so up until the ancestor arc, my understanding of Hiwatari Sr was a regular if somewhat eccentric human. That the Hiwatari family is a branch of the Hikari, one probably focused around being a go-between for the art-centric Hikari and the rest of the world (specifically the police in this case because Dark). Hiwatari stated multiple times he didn’t want Satoshi to die, that’s part of why he adopted Satoshi, and he’s got his own motives for wanting Dark gone. Gets along with Krad (a whole big question mark there, but explainable by a) willingness to use ruthless methods to get rid of Dark that Satoshi won’t use b) willingness to keep Satoshi alive, therefore Krad too). My assumption was that he was a branch family parallel to Satoshi -- in a high level position at a young age indicating some level of genius and a hate toward Dark-- who at some level wished to be the main Hikari line: see again his desire to get rid of Dark through any means, the interaction with Krad that felt similar to how the Niwa interact with Dark (respect, support of goals, somewhat casual interaction) (meaning Hiwatari also somewhat parallels Emiko? As caretaker of tamer-child, but also ‘can’t be krad/dark so put expectations on child’ feel?).
So there’s that antagonistic relationship between Satoshi and Hiwatari, and part of it’s the difference in opinion on how Satoshi should be handling the problem, part of it’s Satoshi’s self-destructive habits, and at some level it feels like underlying resentment that Kei adopted Satoshi on Satoshi’s side. Kei supposingly adopting Satoshi for Satoshi’s sake, meaning either in the period of time Satoshi was raised by his mother, or raised by someone else, he didn’t agree with something going on there-- how Satoshi was taught to view art? That he could change Satoshi’s thinking/actions/survival by adopting him? So much is unclear. But those motivations, the desire to control the future of the Hikari despite being a branch member and complicated feelings surrounding Dark, Krad, and the cultural revolution--and complicated feelings over Kei’s role in the police even--are all speculate-able, human things that build a character readers can kinda sort of grasp.
But then the ancestor arc dropped and it’s.... All thrown out the window because he’s not human, motivations are even less clear, and so many more questions about everything.
And now I’m very ??? New theories: He’s an artwork, probably created by original Hikari or something IDK. Created to support the Hikari but after so many years alive has gone a bit rogue as have so many other old art works we’ve seen in the series. The cultural reform probably has ties to him? Wouldn’t surprise me. Another theory, he’s some sort of being of chaos and is staying around the Hikari because they’re disaster magnets along with the Niwa and he’s enjoying watching the curse unfold. He doesn’t want Satoshi to die because that’d be the end of his entertainment. (yeah, not very probable) And, gah, I dunno. Honestly most probable that it’s art related given the series, but how, by whose hand, when, etc is all a mystery along with Kei’s motives. Real question is how the heck did he get Krad on board ....Tho if you’ve had a few centuries to interact I suppose you could gain a rapport.