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Part 1: Meet Our terrible Intrepid Protagonists!
Well then, let's start a new game, and get going!
Muenzuka
: It's originally near the outside, but for the barrier to weaken this much... damn that youkai, what's she thinking?
: ...This shouldn't happen if Reimu's being vigilant about the border either, should it? That Reimu, I heard she was staying at the SDM recently... So that's why she hadn't noticed? Damn, why me...
: If something isn't done, something bad will happen... Is there anything I can do?
: What's this? Looks like something from the outside world, but...
: ...Yes, with this...
Hmm, very mysterious. Well, Rinnosuke mentioned that Reimu's been lodging at the SDM, so let's go take a look over there.
Scarlet Devil Mansion
I'll get into this more later, but for the sake of convenience, I'll largely be using sprites from outside the game itself. Most of them (for now at least) will be from Touhouvania 2.
: ...
: Hey, hey! Don't just ignore me.
: Can't you see? I'm sweeping the floor right now.
: Oho, good job. I've run out of sweets though, so I'd like you to bring me some.
: If you don't act properly...
: What? Are the SDM's maids always this rude to the guests their master invites? Ooh, how scary.
: Tch...
: So, bring the sweets please. Ahh, the SDM is awesome.
: ...Useless shrine maiden...
Our protagonist, everybody 
Elsewhere in the Scarlet Devil Mansion
: Oh, Sakuya. What are you doing with that many sweets?
: ...Milady, how long is Reimu going to be staying here for?
: Huh, Reimu? What are you talking about? Ahh, yes, when Reimu helped settle some troubles I had, I did tell her she can stay here for a little while. That was about a month ago, I believe. Is something the matter?
: ...She's still here, you know.
: ...Who?
: Reimu.
Back to the first location in the Scarlet Devil Mansion
: Ah, Remilia, long time no see.
: It's been awhile... no wait! Why are you still here?
: Well, you didn't tell me to leave.
: I thought you'd go back! Sheesh, leaving the shrine for a month like that, no wonder no one goes there. Teasing the maids and being so lazy and self-absorbed...
There's nothing to indicate that anything's actually happening, here. We'll just have to take Reimu's word for it, I guess v
v
: ...? If you're trying to escape...
: Wait, milady! I don't know about what Reimu feels, but these tremors...
: This sensation... It's coming from above! Right above the SDM! Quick, open the window!
: Ah, Reimu, wait!
Outside the Scarlet Devil Mansion
: Don't spoil the atmospheric and gitting BGM with your weird "Geh!"s.
In case you haven't been already, I recommend that you give the tracks linked in the locations a listen. Most of them are pretty damn good.
: You've done that before too. ...Anyway, just what is happening here?
: That purple place in the sky, it seems like it's getting even wider. As if it's infringing on Gensokyo's own sky...
: Hey, isn't it... It's sucking in everything around it, isn't it? Look at it.
: Ahh! It's sucking up the SDM's windows and doors! Hey, Reimu! Do something already!
: Er, that's a bit unreasonable I think...
: Well, the frame is big, so it won't all be destroyed...
: Huh? Who's that?
: Reimu, it's me! Damn, I didn't make it in time...!
: Rinnosuke! Why did you come here? ...No first things first. Rinnosuke, do you know something about this?
: Ugh, as I thought, you really didn't notice a thing!
: Shopkeeper! You're speaking as if you know something.
: Would you explain what's happening, if you could?
: Of course, that's why I'm here! Although now it's something I can't handle! If only I got here before this started happening...!
: Well, first of all...
: Wait, about the SDM!?
: Residences as big as that should weather it fine. Right now, our lives are more important.
: Hey, wait! Geez!
: Anyway, leaving here seems to be the best choice, hm? Ahh, just what is going on here?
The screen fades to white here, before going on to the next transition.
Scarlet Devil Mansion
Fraps makes a cameo appearance in the corner here 
: Ahh, Marisa. You're finally here.
: Yeah, I couldn't just leave somethin' so interesting alone. ...So, what's this incident here?
: It's because of that weird thing above the SDM.
: Ohh, you're here too Patchy? That thing above seems like Yukari's doing somehow.
: It's not a laughing matter. Geez!
: Anyway, Marisa. Where are the others I asked you to gather?
: Ahh, yeah. I was gonna call those I saw to come on over here as well, but it looks like everyone already went inside that weird thing for some fun. I didn't see anyone around.
: Ahh geez, if they had come here...
: Hey Kourin, just what is that thing up there?
: That's what I figured I'll explain now. Although I don't know the finer details either...
It seems like a tear in a space interval... that's what it looks like, at least.
: Huh. Something like those FF last dungeons then?
Goony Reference *1
: ...Do you all know about the Hakurei Border coming apart?
: Hakurei Border... the border to the outside world. Reimu, aren't you supposed to be taking care of it?
: C, coming apart? What's that mean?
: Ahh geez, the Hakurei shrine maiden's this hopeless!? Sigh... that seems to be the source of this incident.
: I have a single question. Why were you able to detect irregularities with the border to the outside world?
: The items from outside have been increasing lately. So after investigating, I ended up at the conclusion that something happened to the border...
: ...I see.
: One more thing. Yukari has been acting odd.
: Yukari?
: Shouldn't she be hibernating at this time?
: The one even lazier than Reimu? Is it gonna rain buckets here now?
: Instead of raining buckets, we get a massive hole. I've talked with Yukari before. She's not all that different from a normal person, but... There aren't many people who can affect the border... I thought I'd caught onto something, but that was as far as my investigations went... Anyway, I had to tell Reimu, and she ended up being here.
: I see...
: I don't think it's time to just talk about all this...
: That's true, but there's no doubt of Yukari's dubiousness.
: But what can we actually do about this?
: If Yukari really summoned that, that means she has some plans for it, no? That's just a guess though.
: Even if she's not inside that, I think investigating that space will be beneficial.
: Additionally, it would be good to confirm the safety of those who already entered...
: I shall not be satisfied until I bring fown the one who made the SDM become like this either.
: Alright, then let's-
: ...What?
: ...What did you just say?
: Doing with the SDM as you pleased selfishly?
: Then leaving just like that? Hmm, you though you'd get away?
: Reimu, we're going! You better prepare now!
: Ahh, how troublesome... I don't want to go...
: I'll try to investigate a bit more about this incident.
: Come on Reimu. You have 40 seconds to prepare!
: Bar-
: Shut up.
Next Time
Actual Gameplay!
Dramatis Personae
Rinnosuke Morichika
A half-human, half-youkai with the ability to recognize an object's name and purpose, and the proprietor of the Kourindou, an antique shop. As a half-demon, he has lived for at least a few decades - long enough, at least, to have been familiar with the previous Hakurei shrine maiden. He used to work for Marisa Kirisame's father, so the two have long been acquainted. Perhaps because of this familiarity, Marisa often calls him Kourin, a shortening of his shop.
He is highly fascinated by the items that flow in from the outside world, and often fills his stock with them. He rarely actually sells them though, except for items of equivalent value or better. He's actually the one that built the Mini-Hakkero reactor that Marisa uses for her signature spells, via a blend of magical theory and technology from the outside world. He and Yukari Yakumo are said to be acquainted with each other, due to their mutual interest in the outside world. (She taught him how to use a hookah, among other things)
He's a pretty weak guy (perhaps because he's the only guy), so he's often used as the baseline for an average person in Gensokyo.
Reimu Hakurei
The main protagonist of the Touhou series, the shrine maiden of the Hakurei shrine, and one of 4.5 humans featured in the main games. Her two primary powers are fight, and the ability to call on Gods for power. She rarely utilizes the latter, and the former is something everyone can already do anyway (though in proper Touhou fashion, she turns it into some ridiculous later in the series). She's not at all handicapped by these facts.
Her laziness is kind of played up in this game, but even in canon she puts hardly any effort into anything - but not without reason. As a natural-born genius, Reimu is good at pretty much everything she does, and seems passively receive the blessings of various Gods even without her asking for it. As a result, she's basically never beaten, which is handy, because it's also her duty to investigate any strange happenings around Gensokyo. As Gensokyo is a rather egalitarian "might makes right" kind of society, Reimu often butts heads with troublemakers, but her complete lack of pregidous towards Youkai means she often becomes friendly with those same troublemakers later on.
Marisa Kirisame
The secondary protagonist of the Touhou series, a magician, and one of 4.5 humans featured in the main games. Her powers stem from her magic, which tend towards stars and lasers. She favours flashy attacks and brute force over skill and strategy, which is kind of impressive for a human in a largely Youkai-dominant society. Don't let that fool you though - studying magic isn't for idiots, to say the least.
Perhaps appropriate to her self-appointed rival, Marisa is the foil to Reimu in quite a few ways. Where-as Reimu is a lazy genius gifted by the gods, Marisa is a pretty normal human who obtained incredible magical power by putting in lots of studying and practice (and a bit of practical burglary). While Reimu is the type only to intervene if something inconveniences her or her duty demands it, Marisa tends to investigate things out of interest or curiousity, and sometimes just to beat Reimu there. All in all, the two are pretty different, but are pretty close friends in spite of it.
It's worth noting that, unlike the other magicians of the series, being a magician is her occupation, not her race - Marisa is all human. The details of her family are largely undetailed; her family is apparently still around, but she has cut ties with them for reasons unknown. Hanging out with Youkai and Shrine Maidens seems to suit her just fine, though.
Remilia Scarlet
The final boss of Touhou 6: Embodiment of Scarlet Devil, and a vampire from the outside world. She moved her mansion, the Scarlet Devil Mansion, from the outside world to Gensokyo a few decades back. She has all the fearsome strengths of a vampire, and most (though not all) of the traditional weaknesses - she's got no problem with garlic or crosses, but sunlight or running water (including rain) will mess her up. She's pretty much fine with a parasol, though vv
Despite being 500 years old, Remilia is one of the more childlike characters of the series, in both appearance and temperament. She's got a big ego, a short temper, and a selfish streak, though she can be polite and elegant when she puts her mind to it. She can (and does) drink human blood, but she's known for being a light drinker, so she rarely actually kills her victims.
Her power to manipulate Destiny is one of the more esoteric ones - some fans like to think she can't use it consciously, since that'd be pretty powerful, but ZUN has explicitly stated otherwise. It's stated that anyone meeting her could have their fate for decades to come altered just by passing by her on the street, and that she could turn an ordinary human into a half-Youkai if she felt like it (which is apparently independent of being a vampire).
Sakuya Izayoi
The 5th boss of Touhou 6, and the head-maid of Remilia Scarlet. She is apparently one of the 4.5 humans in the main series, though whether or not she's actually human is debatable. At the very least, it's a mystery how a human came to be able to manipulate time, or wield knives with as much skill as she does.
For that matter, much about Sakuya is a mystery, being the only human to work at a Vampire's mansion. It has been suggested that she was a former vampire hunter that was defeated by Remilia; however, in an act of mercy, Remilia opted to manipulate Sakuya's fate and have her become her maid - this is pure speculation both in- and out-of-universe, however. It's also been noted that Eirin Yagokoro, a Lunarian, seems to recognize her (though Sakuya doesn't reciprocate), and given their shared hair colour, there might be a connection there. Either way, "Sakuya Izayoi" is an alias granted to her by Remilia, and not her real name.
She's mostly cold and courteous as you'd expect, though she does have a bit of a playful side to her that shows up from time to time. Good luck managing to socialize with her enough to see it though - she barely ever interracts with anyone outside of the other residents of the SDM.
Patchouli Knowledge
The 4th boss of Touhou 6, a Magician, and the librarian at the Scarlet Devil Mansion. She doesn't serve Remilia like Sakuya does, but as her friend, she often winds up going along with Remilia's selfish request anyway. She focuses on elemental magic, and is known as the "One-Week Wizard" due to her spells corresponding to the days of the week on the Japanese calendar (Sun, Moon, Fire, Water, Wood, Metal, Earth, in that order, from sun-sat). She's pretty powerful when she gets into it, but her chronic asthma and poor constitution render her often incapable of reciting her own spells.
Patchouli's a pretty calm, quiet person, and definitely the in-doors-type. She's not very excitable, nor very temperamental, and tends towards being the voice of reason at the SDM. She's a bit of a loner, considering she's spent most of her century-or-two life stuck in libraries crafting spells, but she doesn't have any real problems socializing, either, if she needs to. She creates problems and incidents just as often as she solves them, though it's mostly just to stave off bordom for her fellow residents.
Unlike Marisa, Patchouli is a Magician by race, not just by occupation. There isn't much difference, except that Patchouli is technically a Youkai, and thus has a lifespan to match.
Goony References
*1
Lines: "Huh, something like those FF last dungeons then?"
Location: Introduction
Reference: Final Fantasy series
The Final Fantasy series last dungeons tend to be famous for being
part of some sci-fi-like dimension-warping spaces, similar to the
entirety of the strange space.