How about a two-fer! Aya and Nitori.
I did Aya here but i'll also take the chance to throw in headcanon that she may have still have long ago-ingrained skills with a sword, but if she picked one up now, she's more likely to swing it like a club ποΈββοΈ Anyway as for Nitori...
General opinion/How much I care about them: our sworn friend, Nitori π I distinctly remember there was a time where i was aware that Nitori was generally pretty popular but didnt care much for her myself? But thats far in the past because I definitely like her a lot now π I'm happy she just gets to show up all the time, even if a lot of it is just brief cameos in the manga.
A ship I love: gonna play it safe by picking out Nitori/Marisa but it really is super cute how much common ground and shared curiosity/ambition they have with each other, and how maybe magic and engineering are basically the same interest. Nitori's more uptight than expected, so that leaves Marisa to be the one to bring more of the fun to whatever odd business or experiment they're doing.
(also bonus shoutout to nitori/momiji and anyone who wants to imagine the bubbling feelings that may rise from cozy shogi matches despite the various complications of their respective species/classes π)
A non-romantic relationship that I love: it just occurred to me that i've never ever seen anybody do anything with Nitori and Kokoro's newfound partership from acof! i guess i understand since there were so many more interesting and exciting pairings going on in that game, but i thought their brief stint together, analysing and insulting everyone in their deadpan manner was fun.
But actually beside that, its not hard to believe that Nitori's pretty darn well-known across the whole of youkai mountain, and maybe just about every youkai and god has met her at least once? She's not known as Nitori, the Kappa of the Valley for nothing! So it makes me smile to think about her ongoing business relations and funny back-and-forths with the likes of Aya or Kanako.
My biggest headcanon about them: she is the inventor of the optical camoflage suit but the general opinion among anyone outside the kappa (and something the yamawaro especially like to poke fun at) is how inefficient it is compared to just dressing yourself in the colours of nature, and this annoys Nitori a lot...
An idea for a fanfiction I would like to write/read about them: she'll show up in some things i have planned for the far future... But for something about her specifically? I have to come up with one on the spot, hm.... I like to think she's more particular about that 'kappa are sworn friend of humans' thing than any other kappa (even if she usually just means doing business with them) so i'd like to see one of her attempts to be a helpful hero sort to them, even if she can't help but be awkward around humans.
Something that makes me think of them: sometimes when i manage to fix up my appliances or get them to work myself, i think 'wow i could be a kappa' π