Thoughts on Shuten and Ibaraki (separately)?
Really challenging (separately) there since Shuten and Ibaraki are part of the same myth, introduced in the same event, and generally depicted together, but sure, I can work with that. Shuten and Ibuki, however, are part of the same character and have to go together. Also, see this post if you haven't yet.
In 2002, Colorful Tsukihime established that oni come from two possible origins. Shimousa adds a third kind later, but what matters is that Shuten and Ibaraki were conceived as examples of the original two. Unworshipped gods, those who failed to abide to the rules of civilization, and their bloodlines (Shuten) or mutants with power far beyond what falls within the human category, those perfectly unrecognizable as human, and their bloodlines (Ibaraki).
Her backstory is horribly fragmented, even for Sakurai's standards, but assuming we already have all the pieces (and it really looks like we do), I believe it goes like this. The Hii River existed as part of nature, and the faith of the ancient Izumo tribes deified the river spirit (who was also a dragon for some reason) into Yamata-no-Orochi, who was then slain by Susanoo. Orochi's eight heads continued existing as eight mountain gods, one of them being Ibukidouji at Mt. Ibuki. Yamato colonization happens, despite Yamato Takeru's death at Ibukidouji's hands, and Ibukidouji loses her worship. She takes a wholly unelaborated trip to China during the process, and around the late 900s, returns to Japan now with her divinity pretty much fully degraded. After a minor stop at Mt. Sagara, she eventually takes nest at Mt. Ooe, meets Ibaraki and spends the rest of her life as a freeloader in Ibaraki's place until Kintoki cuts her neck.
Ibuki/Shuten has been through multiple self-identities there and displays completely personalities each time, but there are things are a couple of things that are consistent in every version of her. The first and more outwardly obvious is her free-spirited hedonism. Her moral compass doesn't judge in terms of beneficial vs harmful, it judges in terms of pleasant vs unpleasant. Drinking obviously poisoned sake? Fun. No regrets, no grudges against Raikou. Getting her head lopped off by a sexy blue-eyed blonde? Fun. No regrets, no grudges against Kintoki. But what is her criteria of what's fun and what's not? That's where the other consistent trait of all of her personas comes in.
Shuten describes herself as a devoted oni. Despite her lack of inhibitions, she's very loyal and devoted to the ones she likes. You can see that since her debut event, where she orchestrated temporarily giving her powers to Ibaraki, but the extreme extents of her subservience only show for real when you see her multiple personas. Ibuki/Shuten is, above all else, loyal to the labels assigned to her. She will be what she's expected to be, at all times.
Ibukidouji originally identify as a god, and while this is not a timeframe ever covered on screen, in a few moments, mainly through her Heian portrayal and her costume, she shows her ability to behave regally and imposingly as expected of a god. She also assigned brutal trials to Saburou like he believed a god should and did the same to Dorothea during his own tenure as a Divine Spirit, but I don't know which of the two started this cycle. There's also the one time Yamato Takeru got fatally owned, but we get surprisingly little detail on that.
Ibuki's 1st Ascension aka Tinybuki is, as shown on her interlude, Ibuki with her divinity pretty much fully degraded. It's Ibuki/Shuten at her most serious ever, acting less like a god who does what it wants and more like a part of nature teaching young Kintoki its rules. It's an interesting phase where she lost the freedom of her god label and has yet to discover the freedom of her oni label, so she's kinda just vaguely bitter and no-nonsense. In different terms, it's after she stopped being loved by many and had yet to come to terms with the liberating idea of being the enemy of many idolized by a close few.
Then Shutendouji is the textbook oni. A walking disruption of human normalcy. Shamelessly depraved, violently monopolizing of anything that piques her interest, selfish as selfish can get (superficially) and easily prone to harming, killing, and eating people. An oni so oni that Ibaraki fell in love with her ability to perform the oni label. More on that in Ibaraki's half of the post.
But then Assassin Shutendouji doesn't eat people because her new label is Fujimaru's Servant and Fujimaru doesn't want her to do that. Then she makes a silly magical girl persona for Ibaraki's sake. Then Fujimaru's Ibuki just takes it a step further and adopts a whole new persona as a nice lady from Fujimaru's era. Compare and contrast to the Douman-summoned Ibuki, who stuck consistently to her most divine persona because Douman only wanted power out of her.
Heian's ultimate self-sabotage gets funnier the more you understand the characters, honestly. Douman gave up last minute on making Ibuki a new Beast because he couldn't trust that Ibuki would remain loyal to CHALDEAS like he is, but Ibuki is all about being who others want her to be. And Douman failed to realize despite her use of absolute loyalty to compensate for her lack of a more self-originating personality being something that Douman shares with her identically. I've talked about this before.
Her last major identity is her summer form. Arctic Summer World and her profile phrase her change into a Berserker Saint Graph as "redefining her self as a goddess of summer" and iirc there's even some mention of dismantling and rebuilding herself in there, making the concept feel more literal than it sounds. It shows how radical Ibuki's label swaps are, if true. There she tries to make an eternal summer for Fujimaru to enjoy (mixing her labels as a summer goddess and as Fujimaru's Servant) and then asks to be pacified in combat when rejected (mixing her Berserker and oni-god labels). It's only when her main collaborators Skadi and Douman tell her to stop that she does, because Ibuki/Shuten always is what the people she cares about want her to be, and this reaction proved she was using a persona literally no one wanted.
Then there's Samurai Remnant portrayal that's just weird. She mixes and matches traits from all her versions in FGO, with no intention of trying to be what Yasuhiro wants her to be. I guess having an identity independent of her Master is a that's a mark of impartiality characteristic of the Ruler class, but I think I'll have to come back to this one after the Ordeal in April.
Still, there are a few characteristics that are wholly unique to Shutendouji. Things that clearly came as a product of time and experience rather than as her nature to comply to expectations. The first being that Shuten is far more protective of her Shuten identity. Ibuki is willing to immediately throw her more divine persona to be Fujimaru's sweet neighbor figure, but Shuten, even becoming Oni Cure for Ibaraki, remains very recognizable Shuten. Also, when the Ibuki side of her Saint Graph tries to take her over in her Interlude, she very aggressively says she's Shutendouji and her older self doesn't get a turn on her Shuten Saint Graph. Ibuki and Shuten are otherwise very unshakably flippant, so Shuten's rejection of her past behavior stands out as her rare moment of vulnerability.
The other trait exclusive to Shuten is her clever talent to attend to her friends' needs. Ibuki is described as a corrupting force that ultimately kills everything she touches, but Shuten is very helpful to the one she cares for in very thoughtful ways, while ensuring the circumstances still make her look evil.
Her debut has knocking herself unconscious for the whole event give Ibaraki the power she needed to prove herself and overcome her complexes about not being oni enough. The pair's dialogue is designed to resemble interactions between an employee and an abusive boss, but in reality, Ibaraki is the real boss who only lets Shuten have control because she loves Shuten as an example of a what an oni should, and takes Shuten's fake authority as a lesson that lets survive leading her army of oni. She orchestrates the whole first tower event to help Tomoe sort through her denial of her oni blood while under the pretense of self-interest in human-oni relationships. And so on.
Disclaimer: I hereby promise that I really, really tried to fit a "snake sheds skin" metaphor to describe Ibuki's ability to easily discard personas, but I couldn't a proper place for it in the text. Sorry.
Ibaraki, despite being the more interesting part of the duo, has a lot less things to talk about. She's surprisingly straightforward. Her backstory is as fragmented as Shuten's, but her fragments are clearly meant to be a neat puzzle the player is meant to solve. I have a past post about it. Plus addendum.
Ibaraki's deal is that she's a naturally conscientious, diligent, non-proactive, and cowardly kid trying and failing to be live up to the capricious, arrogant, and decadent image of an oni as described by her dear mother. Ibaraki was raised without freedom and instilled with the idea that should embody freedom as an oni. But when she kills her mother and begins living by her oni identity, she doesn't know how to be free.
In her very human-like way of thinking, she becomes the leader of a band of oni, something unprecedented because a true selfish oni would never think of grouping together. Through that she discovers Shuten and grows enamored with the ex-god's ability to embody the platonic ideal of an oni. So while Ibaraki remains the leader on paper, she made Shuten the freeloader who call the shots. As someone who processes the world as rules that need to be followed, having a role model like Shuten around is the most reassuring thing ever.
What Ibaraki can't possibly know is that she struggles so hard to be an oni correctly because there's a complicated human behind her oni mask, while Shuten, as shown mainly through Ibuki, can perform any label effortlessly because there's a blank slate behind her multiple masks.
