This chapter 42 finally clarifies the gist of her grudge + possibly why Suika is helping her : she's was an extermination specialist (that gave away her humanity, but we'll get back to that later) affiliated to the Hakurei Shrine (as the Miyadeguchi family served under them) that fell to the youkai scum. Also kinda expected her to be a Hakurei no Miko proper rather than a random miko affiliated to the Shrine, but that's more of the same.
The most unexpected and interesting part to me was the fact Miss Miyadeguchi was actually a Jinyou, a human who became a youkai, AND even partook in attacks on human : meaning that she switched sides, from humanity to the youkai one. Furthermore, her transformation from human to youkai was not just motivated by self preservation, but also by some ideal related to how the relationship between youkai and humans (particularly the extermination specialists) was to be (an ideal that she may have shared with Suika). Unfortunately we don't get insight into what she thinks of youkai in this chapter plus the additional explanation (besides survival) for why she'd become a youkai.
The problem, however, is that after her death the world didn't quite progress in the way that she might have hoped for : the Hakurei Shrine (and the Miyadeguchi family under its service) decided to side with the youkai. Nowadays we know that youkai would have pretty much disappeared due to science's progress, while the necessity to keep some humans alive means that they can't properly use their power and slowly lose that too : so the Hakurei Shrine's intervention was definitely VERY favorable towards youkai's continued existence, while the current setting's incident management just keep them strong and doesn't feature proper exterminations (Reimu / the Hakurei Shrine basically keeps them around)
This presents a pretty unusual view of the setting, where its presented that the Hakurei Shrine is le bad, as Miss Miyadeguchi pretty much call them traitors towards humanity (even roped her family into that). The interesting thing to me lies in the fact that Miss Miyadeguchi HAS betrayed humanity herself (for self preservation and additional unclarified human-youkai relationship related reasons) : what is her point of contention against the Shrine siding with youkai ? And what is the world of youkai that she and Suika hoped for before the Hakurei shrine gone and done it ? Cuz strictly speaking, current Gensokyo is pretty much a haven for the youkai and is relatively safe for its native human population, despite the humans living in far inferior conditions when compared to the outside world AND even having the youkai scum infiltrating their society at every level : there a suspicious kind of peace, but humans definitely have the short end of the stick.
So imo, she's probably super pissed cuz not only she pretty much died for nothing BUT also because her "side" of the conflict sided with the youkai instead and benefited them greatly even in current setting touhou : even nowadays "youkai extermination" can be considered to be a farce. Reimu bears the brunt of her grudge cuz aside from the whatever previous disagreeable reasons there may be, Reimu IS the current face of Hakurei Shrine AND even plays a hand in what angers Miss Miyadeguchi cuz she sanctionned the spell card rules ... Presented like this, it makes more sense why she targets Reimu and follow the order of incidents that follow the spellcard rules in their resolution ...
As far as the human-youkai relationship goes, FS illustrates it best, and it suddenly appearing in this manga this late doesn't really allow it to be properly fleshed out in this particular manga. However, FS is more modern setting, while Miss Miyadeguchi's days and worldview is about a century or so earlier. So I'm a bit bummed that we only get this SUPER late, and don't even get to see what exactly is the world with youkai that Suika and Miss Miyadeguchi envisionned (or Miss Miyadeguchi's second reason to give up her humanity).
If we go back to the VERY old part of the setting regarding the start of Gensokyo, we have a land haunted by youkai as well as verious extermination specialists that gathered there (and eventually settle the place). IF there was neither Hakurei barrier nor the current arrangements regarding human village and youkai, the youkai would have eventually disappeared : bit of a speculation, but maybe the extermination specialists understood that they'll fall into complete irrelevance and into obscurity once the youkai are gone and decided to take action in order to secure their prosperity too (would suck to be left in some godforsaken woods once the job is done while the rest of the world move on).
And another additional speculation regarded to the balnce between powers here, but regarding Jinyous in general, surely the reason why their existence is a massive threat towards Gensokyo is because it means one less humans AND another enemy of humanity on one side (would still need to farm fear of their own, as seen in Miss Miyadeguchi having attacked humans despite decidedly being on their side, while also presenting an alternative towards living in fear of the youkai for the humans AND less humans to farm fear of : if transforming into Jinyou was presented as a proper option to the wider public, the human society could probably collapse (as just any of your neighboor could become a hostile monster), while the youkai could lose their control overthe humans as well as the fear they need (as becoming a youkai may become cool, they become less scary). Not to mention the increase of power Jinyou gets + the uncertainty of their agendas : what will they do ? even Miss Miyadeguchi herself was evidently pro human despite attacking them occasionally. Dunno if the potential forJinyous have something to do with Miss Miyadeguchi's view, but it wouldn't be surprising considering how she rejects the current state of things.
So yeah, all in all, a pretty interesting chapter if you read a bit into it. When I saw that she was actually a human turned youkai, I was like "wait a minute, isn't there a MORE interesting story to tell here ?!!!" Cuz aside from being acquainted with Suika early on and turning into a youkai on her suggestion, I feel like her end is not as interesting (cynical, I know) as what her life, ideals, and motivations that led her to become a Jinyou would be. Also VERY disappointed that the first time we get to view a proper Gensokyo backstory, it's just a few dialogue scenes tbqh.
Anyway, the ending of the chapter 42 shows Reimu doing quite a bit of thinking regarding Miss Miyadeguchi's circumstances and resentment. I quite like the usual depiction of Reimu where she's insensitive, cynical and pretty much cold hearted (FS Reimu superior), but I'm quite open to seeing a human side as well. Like ... is she feeling *gurgle* pity here ?! A bit weak for the crimson slasher but sure why not. What is she gonna do next chapter ? Whoop Miss Miyadeguchi's butt into the afterlife and be done with it ? Or have some kind of heart to heart conversation about current state of things ? Cuz after all, this is still touhou, and we have NEVER seen a 2hu removed from the setting (nor will we ever). Whatever's gonna happen next, imo we'll have the grudge towards Reimu significantly scaled down, but she'll still hold a bit towards Hakurei Shrine to keep existing (a bit like Tojiko).
Also things look like they're coming to a close, but we still haven't seen the SDM crew make a move, and the same goes for Yuugi and Suika (I hope their role is not just gonna be to open a hole and that's it), AND we still need a certain title character to do something.
Minor takeaways :
- An additional title for Miss Miyadeguchi "Traitorous human-youkai shrine maiden", making her the most titled character to appear in one work
- First bunch of youkai who actually eat a humanish person, also very poor table manners
- Jinyou having a 100% fatality rate (2 for 2) now, making it the most lethal race to be in the series
- Actually still don't know what Miss Miyadeguchi's abillity is, huh
- A single line of dialogue for Miss Satori