headcanon; I’m gonna round up and rewrite my Arceus headcanons here. Because I can.
Preface/let’s get the less complicated part of this headcanon out of the way first: Arceus has never really left the Hall of Origin. The pokemon “Arceus” is a projection, a vessel, but it isn’t truly Arceus. The closest any humans have been to actually, truly perceiving Arceus as in Arceus are the Legends protagonist(s) and the conquest protagonist(s).
Likewise, the Hall of Origin can only be accessed from two places: Spear Pillar, where the world was first created (and, consequentially, where the “line” between our world and the Hall of Origin is the thinnest), and Ransei, the liminal gateway he placed here to really just make the whole thing a bit more convenient for his divine children. That’s it.
So onto the complicated part: Arceus, existing outside of the confines of time and space, does not really exist in the way, uh, basically everything else does (exception being maybe the creation trio, but even they exist on a Slightly different wavelength than Arceus.) As such, Arceus also doesn’t really experience, like, emotion the way anything else does.
Well, sort of. Arceus can, in fact, feel emotion. It just has to be an extremely powerful and borderline overwhelming emotion. As of yet, he’s only ever felt two. The first: betrayal. Towards Giratina, for its defiance, and towards Ransei, for… the same. Two peas in a pod, these two!
The second is love. (lol gay). Specifically, towards the first Emperor of Ransei, Sewia Mochihito. (Arguably he has experienced a third, Admiration/Respect, towards the DPPt & Legends protag(s), but we can table that as it’s debatable whether or not that’s really an “emotion.”)
It’s hard to say exactly why this is the case. Perhaps it’s just because Mochihito was the first human Arceus ever allowed himself to get close to, as his hero and perfect link partner. It’s possible that he only ever felt this way because of the link between them, as it is an empathetic bond, after all. Nevertheless, Arceus, as gods are wont to do, fell hard. When Ransei slowly begun to break apart, he offered Mochihito ascension (a word I cannot spell to this day), basically saying listen babe I can make you a god you just gotta say the word, but Mochihito turned it down, fearing the loss of his own humanity if he did so. So Arceus begrudgingly accepted, and instead offered him some protection in the form of the now very aptly named Murder Crown.
This would prove to be maybe a bad idea, because this is also what ultimately ended up getting him killed. The story goes as told from there: Arceus banishes the island, dubbing it “Ransei” (lit. ‘turbulent times’. We have a game in the conquest group chat about finding that phrase in sengoku media, it’s very fun) and swearing he would never return until it was reunited under one banner. In addition, he cursed the souls of Ransei’s people to remain there forever: those who died in Ransei would always be reincarnated back into Ransei, in a never-ending cycle of spite.
The one and only exception to this rule, was, of course, Mochihito. The Emperor’s soul was spared from this cycle, granted immortality to serve as his vassal for the rest of time, be generally gay, etc. This went well enough for a few centuries until Mochihito expressed his desire for reincarnation as a means to return to Ransei and try to fix things, as Arceus was incapable of doing so himself due to his vow. Arceus was not fond of this idea, as it meant losing Mochihito forever. However, eventually, he obliged, allowing Mochihito to be reborn as a new soul.
This reincarnation was Hiroko. (Actually, Arceus calls her Kaguya. Make of that what you will.)
As such, Arceus considers Hiroko to be the child of himself and Mochihito. And, like, biologically speaking, it’s unclear if she actually has any actual parents? Like, maybe? Mind you, Arceus didn’t really think that unification was possible, so he was not super thrilled about ANY of this, but he could at least rest assured that she’d come back eventually, even if he was right and it was impossible. (Turns out, he was not right. Way to go, Hiroko!)
You may be wondering: Why didn’t he reveal himself to Hiroko sooner? Well, he wanted to honor Mochihito’s wish for this reincarnation to be able to live freely, without interference on Arceus’ part. Plus, that would kind of be cheating on the whole “not coming back until the war ends” thing. So he only let Hiroko know that he was following her efforts after Ingis, just to give her a little push in the right direction.