Recently, I read through a fairly extended not-quite conversation, but meta-monologue about the writer's disappointment with the fact that a particular character introduced in a relatively recent game (half a decade ago this year) hadn't gone on to become part of the central cast, but had simply appeared in a couple of works that had tied into her game's release and one that summed up the previous few games in the series and tied up a few plot threads, but not many.
It would be cheap and easy to say that this person misunderstands what Touhou is and how Touhou is produced and why it's produced in the way that it is, and this is perhaps true. But what's more relevant to me is that this particular shape of understanding the kind of families of fictional works we call "franchises" which tend to be multimedia and where many if not most members of the family don't have the stamp of the original creative team's direct involvement is one that appears to be a consequence of a rather twisty approach to understanding fiction, art, and media.
So let me first explain what Touhou is and what it does. This is a fairly difficult but enlightening process because- just what is Touhou? You might say "it's the thing with all the girls that used to be all over Pixiv", or you might say "it's a series of shoot-em-up games", or specify "top-down danmaku shoot-em-ups", and even the latter elaboration would be facially wrong.
Because the first Touhou game ever, Highly Responsive to Prayers programmed by the series' creator, Jun'ya Oota, who prefers to go by ZUN, was an Arkanoid derivative. However, it was released alongside a top-down shooter in the danmaku genre, the sequel, Story of Eastern Wonderland. The sixteen additional "mainline games" with an integer numbering have followed Story of Eastern Wonderland's genre position.
But there are six "side games" (with a decimalized numbering) which are fighting games, and which have involvement from ZUN, though they are primarily made by Twilight Frontier, and there is another game from this production process which is a platformer. (And an additional six made by ZUN which are variant danmaku shooters.)
So it's a series of video games, then. But it isn't, because ZUN has stated in interviews, perhaps puckishly or modestly, that the games exist to provide a reason to compose music. So the sixteen music CDs ZUN has released, plus the seven Twilight Frontier has released as OSTs of their games, are in some fashion core to the creator's understanding of what Touhou is.
Then there's the prose fiction (46 short stories and a short fixup novel), the comics (251 total chapters so far), the factbooks and artbooks (three of the former, two of the latter), and an official fanzine that combines material by ZUN with material by other creators... and this in turn leads us to a more important angle of understanding what Touhou is. Touhou is a series of doujin products. The games are straightforwardly doujin software. They exist in the context of Comiket and in the twilight world where independent/self-published original work coexists with derivative works that are quasi-legal (though there have been shifts towards more explicit legal protections in recent years).
"Official" and "fanworks" for Touhou are both within the category of doujinshi. ZUN fosters fanworks, collaborates with fanartists to illustrate print works, and even the dedicated people of the English-language wiki are uncertain of how to classify the comic Inaba of the Moon, Inaba of the Earth, which had loose, uncertain involvement by ZUN but also has the imprimatur used for related prose fiction and comics ZUN wrote directly. Their answer is to note that "the canonicity of this manga is highly debatable".
So that's what Touhou is- a nebulous array of multimedia works, which in the "fanwork" side of things also include oil paintings, multiple independently produced original video and original net animation works, multiple board and card games... it extends its reach into a great many domains of the arts, before getting into any cultural influence it has had in a quarter-century of existence. The boundaries between fanworks and official works are blurred.
So part of the function of Touhou's core "official" works is that the characters that are introduced in a game are there for you to pick them up and use them if you like them to make more Touhou. This means in turn that characters only receive a limited quantity of official attention even in "expanded" works like comics and prose, because ZUN is simply another creator of doujinshi, worthy of respect but not in an exalted place of separation from other doujin creators.
But this runs somewhat counter to how most people interact with multimedia franchises, which is to separate out what is "canon", what is fanworks, and what is inaccurate or unworthy official work. And the tragedy of canon is that when confronted with a collection of media that don't make use of this hierarchical principle, people grow frustrated with the lack of hierarchy, or take their frustrations with the general trend of fanworks and displace them onto the creator for not ordering the universe of artistic creation properly. Which is a true shame, a kind of fetter on the mind which I find unjust (even as I am not truly free of it myself- I grew up involved in the fucking Star Wars fandom with "G-canon" and "T-canon").