(Wall of text ahead. I'm not gonna do a tl;dr.)
A lot of characters just fat out don't need any (ex.: fairies such as Cirno or The Fairies Of Light) or don't have any (ex.: Benben Tsukumo & Yatsuhashi Tsukumo (note: as humanoid youkai)).
So, a lot of youkai just don't need any period. However, there ARE some youkai that would be interesting to know about.
The whole Scarlet Devil Mansion cast, imo, could use some development. In fact, they are the single biggest entity in touhou that could use devlopement, period:
- Why does Sakuya pose as a 16-17yo Human maid when it's known that she had known Remilia for "a very long time" which implies to be longer than the age she pose as? And etc. Because, she is the one with potentially the most background.
- What is the family background of the Scarlet SIsters? They are clearly not Japanese, either.
- When and how did Remilia befriend Patchouli? The magician is clearly not Japanese descent either...
- Same as above but for Hong Meiling.
- Concerning Hong Meiling herself, I bet there's a huge possibility that she used to be human at some point and turned Youkai for reason. I guess that you could develop on that.
As for other characters... going 1 one by one...
Rumia: It's said that she's can't touch her ribbon in her hair. Why? That's the only thing of interest needed about her.
Cirno: See above.
Letty Whiterock: We haven't seen her at all since PCB. But... She's a Yuki Onna and, there's no interest in knowing a background to a legend we already know about.
Chen: We know the difference between Nekomata and Kasha. And besides that, we don't really need to know why she became's Ran shikigami.
Alice Margatroid: Yet another non-Japanese descendant. She was "revived" from the PC-98 era into a new canon. However, there's a lot of unknown about her. Mainly her book. But also, she is the most popular character to be believed to be at the center of "Dolls In Psudo Paradise". So, she could be in a very interesting character to get BG info from.
Lily White: Oh hey a fairy.
Prismriver SIsters: We already know that they were conjured by Layla, now deceased. So, the only thing of interest, because they are clearly not Japanese descent, is how they came to Gensokyo.
Youmu Konpaku: Youmu herself, I think we know enough of her because, clearly, she hasn't had a lot of life experience. However,
Youki Konpaku: Now, that's something to know about. Youmu's grandfather, Youki. He's only been mentionned and has disapeared to God knows where.
Yuyuko Saigyouji: I think we know the necessary stuff about her already. But the only thing of interest worth of devlopping is why is she friend with Yukari in the first place. But even then...
Ran Yakumo: As much as I'd like to know more about her... errr... about a living computer? I think the answer should come with the next subject.
Yukari Yakumo: And if there was one single youkai defining mystery, that would be her. The striking ressemblance with Maribel Hearn, her supposedly "real name" (if any)... Heck, she may very well be the definition of a boundary. After, the SDM residents, she takes the cake. She could probably have a whole anime adapted to her background.
Suika Ibuki: She has kind of a mystery aura around her for some reason. There's something I'd like to know about her but more specifically, about the "The Big Four Of The Mountain". There is one character unaccounted in these four.
Wriggle Nightbug: We don't really need to know anything here.
Mystia Lorelei: Same as above.
Keine Kamishirasawa: It's implied that she became what she is and wasn't born like that. So that'd be interesting to know why she is like that.
Tewi Inaba: She is probably The White Hare Of Inaba. So, that's enough if it's confirmed.
Reisen Udongein Inaba: A lot is known about her but there's a hole in her story and it's the environment as to why she came to Gensokyo and discovered Eirin and Kaguya. Some would say is the "Lunar War" when the US landed on the moon in the 60's but heh...
Eirin Yagokoro: To be perfectly honnest, I think there's enough background around her.
Kaguya Houraisan: She is based on the popular Japanese legend, you know, The Tale of the Bamboo Cutter. So, to be fair, that's mor ethan enough!
Fujiwara No Mokou: The only thing that could make an interesting tale about her is the gap between when she drank the Elixir and when she discovered Kaguya in Gensokyo. Even then, it was briefly explained and I think it's enough.
Aya Shameimaru: She has quite the spotlight these days but, to be honnest, we don't need to know her background.
Medecine Melancholy: Oh hey, a doll youkai/tsukumogami. No, we know enough of her.
Yuuka Kazami: I can't lie, there's a sense of mystery around the Windows canon of her. But is it really worth it? Nah...
Komachi Onozuka: There's not really a lot surrounding her and, to be honnest, she doesn't need much more or any more background.
Eiki Shiki, Yamaxanadu: She's like Komachi. Except for the fact I'd like clarification on the "don't mess with her" thingy-majiggy.
The Aki Goddess sisters: To be honnest, they don't need more background. They are what they are.
Hina Kagiyama: Same with the Goddess of Misfortune.
Nitori Kawashiro: Imo, I'd some info on her peers that appeared with her in the mangas. Not herself. We know enough.
Momiji Inubashiri: *cough cough* OK, I'm a fan-boy of her's if that's not obvious enough. However, what I'd like is not BG info. What I'd like is for her to be taken seriously. "Petty Patrol Tengu". Really? But I must admit that she got some spotlight very recently though and I'm glad for that. Also, can we fix the ears concistency please? Pleeeeaaaaaase?
Sanae Kochiya: It's hard with her. A part of me don't care, but another part is curious. But the part of me that doesn't care is kinda overwhelming though. Sorry Sanae fans.
Kanako Yasaka: Goddesses! To be honnest, from what she's based on, it's enough. However... the only thing we don't know about her is how her abilities work.
Suwako Moriya: Oh god.
Iku Nagae: To be honnest, it's not her I'm interested in but the Dragon's Palace and the Dragons themselves she seems to borrowing her power from.
Tenshi Hinanawi: I think it'll be boring to know her background.
Kisume: Nah.
Yamame Kurodani: Neither.
Parsee Mizuhashi: We know her from what she is.
Yuugi Hogashima: Yep. It's one of the four divas. So, I'd like to know a bit more about that.
Satori Komeiji: As much as I'd like more, it's really not necessary.
Rin Kaenbyou: Nah.
Utsuho Reiuji: What I'd like to know is a way to remember her damn name.
Koishi Komeiji: Who? (Ie. Not necessary, she had quite the apparences these days)
UFO cast (except below): Taking a shortcut here. They are either unncessary (Nazrin, Nue...) or we know enough (everyone else).
Kogasa Tatara: She has no surprises for us unfortunately.
Everything in Hisoutensoku is... huh... no. Very unncessary.
Hatate Himekaidou: Like Aya, I don't need more bg info on her. Also, she should upgrade to a smartphone. It's more useful and Google Image is more usable on that.
TD cast: Most of them as their background covered. However, there's one thing about Miko that's bothering me. If she's really based on Prince Shotoku, why is she a girl? How did that work? And all the questions related to that.
Mamizou Futatsuiwa: She has a lot of spotlight recently but she has that mysterious aura. I'd like to know what she was doing in the Outside World before she was called-in in TD.
Hata No Kokoro: Again, we're having cases of "no need".
DDC cast: Shortcuts! None of them offers an interesting backstory that is not related to the events of the game. Maybe Sekibanki and Kagerou Imaizumi could use a bit but even then. Everything stems around Seija Kijin and her nature.
Sumireko Usami: She's very much at the center of the attention these days because of, well, spoilers. Anyway, the only thing right now that scracth my BG OCD is her relation with Renko Usami but I think is mostly likely to be solved from Renko's side rather than Sumireko. Because "time".
LoLK cast: This incident is not even fully resolved and we are in the midst of knowing more. Especially with Hecatia, Clownpiece and Sagume. The rest is pretty much self-explanatory or no need for. Especially the moon rabbits. We don't care about their long military history or mochi-pounding days. That's just me though.
Rinnosuke Morichika: The guy is full of mystery. Being the only major male in Touhou (I know there's other minor males but he's the only MAJOR one), I'd like to know his roots. How does one breed with a youkai? So, his parents is a point of interest in his background. Also, the fact that he seem to have live outside the barrier but can't seem to remember any of it is another point of interest.
Kasen Ibaraki: Just say it you're an oni already. If that wasn't obvious enough. Oh and she's the 3rd (but unofficially) of the Divas.
Kosuzu Motoori: Heh... I think we're going to know something soon enough that'll change a lot of stuff not only about her but about the whole franchise. So no need for BG. We need FG xD
Hieda No Akyuu: We don't need to know about her and her past self. What she is defines what she was all about.
Renko Usami: Tbh, the only think about her that triggers me right now is her link with Sumireko.
Maribel Hearn: See what I said about Yukari. They're likely related in some way and it's making my head spin from all the possible scenario possible. Hopefully Zun expand on her with his next CDs.
...
But now, the protagonists themselves... Reimu and Marisa. Both of them have a very vague past (before EoSD I mean). And it'd be amazing to explore to be honnest. I could say more, but I'll stop here because I've already spent several hours writing this.
Background info, in my opinion, is the weakest link to Touhou in general. You've got this great cast of characters and, a lot of them, just has their primitive first appearance and cameos and... that's it. And it's always expanding. However, what would be more interesting is the one-off things that are mentionned such as the Grassroot Youkai Network. And this includes palces too. Makai, for exemple, was mentionned in Windows canon but never really explained. We only got Hell info from Hecatia a few days ago (and that was welcome! Loved it!).
So, that'll be it for me. If I think of any more things, I'll add at a later point.
P.S I'm not sorry for the occasional puns I've made.
P.P.S Note that all of the above is my opinion. You may or may not agree with it. Don't slap me for having an opinion. Thanks.
