Someone must've used KimikoMuffin's Random Touhou Pairing Generator, because this is way beyond "The Most Unlikely Couples".

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I remember that chapter in the Inaba manga where Mokou abducts Udonge to "eat her". With some leek.

If they're from a same game, how is it unlikely? It makes perfect sense to me if Mokou met Reisen along her duty as a forest-guide and Reisen's duty as Eientei's errand girl.
Then, Reisen came to Mokou to seek sympathy as everyone in Eientei was bullying/teasing her...