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Benikage was really the ultimate role for Yumeoji Fumi because she's simultaneously the most loyal and the most deceptive person in Onikurenai. On the one hand, she betrays her closest friend, on the other, she's loyal enough to her clan to finish her duty no matter what. (cont.)
This reflects Fumi belonging to two families- her natural one at Siegfeld and her found one at Rinmeikan. Initially, it seems like Fumi's arc is the opposite of Benikage's- she betrays her biological family and sides with the found one. But I feel like AA might add another layer.
If we take into account Yuyuko's lines to Rui about Fumi being as convinced about the 'war' as Tamao is, then we can assume she's still loyal to her. But Fumi, of all people, should know how toxic competition can be, how badly it hurts people. So why's she on-board with the plan?
Her character in AA is also described as 'resigned' to Death's arrival- not happy about the apocalypse, but grudgingly accepting it, because closing her eyes won't achieve anything. This makes me wonder if Fumi is obeying Tamao's orders on principle rather than wholeheartedly.
Mei Fan also mentions that she's become distant to Shiori, as if the divide between their schools is not just a matter of location, but a far deeper rift. Just as loyalty to one's family and lord was paramount in Onikurenai's time, so Fumi seems to place Tamao above her feelings.
If Fumi abandoned her dreams after escaping Siegfeld, then Tamao saved her and gave her a new lease on life, a life that she owes to Tamao alone. And I think that she fears alienating Tamao more than anything, so she's going along with the plan to avoid being abandoned again.
The best part is that the Benikage metaphor holds either way- if she opposes Siegfeld and Shiori despite wanting no part of it, she'd be putting her lord above her wishes. And if she turns on Tamao, she'd be betraying her closest friend for her original 'clan'. Very intriguing.
Lastly, an important part of Onikurenai is that Benikage's betrayal is not a shock, but foreseen and even sanctioned by Onikage, who knows the value of duty. Akira (the former best friend and superior) has largely forgiven Fumi and encourages her to do her best with Tamao.
This sets up the Rinmeikan vs. Siegfeld plot pretty well, but Tamao playing Onikage in the past also makes me think she might just be betrayed in turn, and have to deal with accepting Fumi's reasons and motives for the matter. It'd be an interesting dilemma for her to face.
As a bonus, Onikurenai basically foreshadows Rinmeikan's entire Arcana arc. Rui, who played Onikage, is forced to fight Yuyuko, who auditioned for Benikage. But Yukko also says Rui is a more natural fit for Benikage, which checks out, since she betrays Tamao's Onikage. Whew.
Oh, and the event doesn't specify who Ichie plays, though she mention that she 'likes a challenge' when Rui tries out for Onikage, suggesting that she went for the same role (who fits the snippets of leader Ichie we see). Her final role is Raven, an Iga (Benikage's clan) ninja.
So whoever Benikage ends up being, whether it's Fumi or Rui or Yuyuko, Ichie shall side with them, and be considerably more decisive than the 'betrayer', encouraging them to do their duty (which sorta fits her role as the Sun).
*Holds head in hands* Oh, Rinmeikan.
*Holds head in hands* Oh, Rinmeikan.
