Touhou Suichouka ~ Lotus Eater-tachi no Suisei - Ch. 60 - Sake is the Best Medicine (Part 2)

[syncswim]

Reimu has exterminated exactly one youkai ever (RIP fortune teller guy), and it was because his existence as a human from the village that turned into a youkai was a threat to Gensoukyou's continued existence. This is notable because it has to do with what Reimu's job actually is (maintaining the barrier and current balance) vs what her job is generally perceived as. It's probably never going to happen to begin with because ZUN doesn't usually commit to big changes in the world, but if Marisa did turn into a youkai she and Reimu would probably still get along more or less fine. Marisa already lives apart from the human village and has about as much interaction with the villagers as the actual youkai that are constantly hanging around it do (Mamizou, Aya, Reisen, etc). See also: Sumireko technically being a youkai after the events of Violet Detector, but still hanging around Reimu like usual.

Yeah, the logic doujin creators have followed since forever is that Marisa becoming a youkai would cause an instant confrontation with Reimu because of how much it would disrupt the balance of GSK. Any human at all becoming a youkai is a threat (because more people will be tempted to do the same from then on), never mind a super powerful human like Marisa. But now that you explain it, I'm thinking about how Okina tried to make Marisa into her own Reimu, which would have been a huge shift to the balance, but no one seemed to care that much.

 

[Killbornbloodbane]

Reimu has exterminated exactly one youkai ever

She exterminates one other that being the owner of the salt mansion who had been fully possessed by a Headless horse/umatsuki yokai in Forbidden Scrollery where it is not shown, however it is stated by Momizou what Reimu did. So, she has confirmed two exterminations not only one. There is even a funeral mentioned and shown for him in said manga volume 5 chapter 37 here on mangadex is where most of this is shown.