I am nearly 4 late, but allow me to share my insight.
Aya makes Reimu a tsundere. Reimu constantly goes out of her way to show just how much Aya annoys her to no end, and yet she will team up with her, she will help her out/give her information with little prodding, and she will go out of her way to look good in front of her (see Lotus Eaters 71). And Aya, for all the times she tries to get sensationalism at Reimu's expense, does seem to have respect and a small degree of reverence for Reimu, both as the Hakurei Miko and just as a person, but she's also not going to let her kinda-sorta crush on Reimu get in the way of journalistic integrity (again, see Lotus Eaters 71). This could just be bias from the first AyaReimu doujin I ever read, but I also think both are fundamentally lonely people despite the big circle of friends they both keep, and I think they both can find solace in each other.
Ultimately though, I think the core of AyaReimu is "Trust." Reimu eventually gets close enough with Aya that she shares a deeply personal bit of information about herself, knowing Aya won't just turn around and publish it for all of Gensoukyou to see, and Aya can turn to Reimu as a source of comfort and security when the endless grind of tengu society gets to be too much for her (I don't know how accurate it is, but I have this image of tengu society in Touhou being one of endless competition full of metaphorical cutthroats and backstabbers).
That's how I've come to see AyaReimu, anyway.
last edited at Mar 10, 2026 3:56AM