Tanjiro's kindness softening Kanao's conditioned detachment ends up being the framework for like ninety percent of their fics. Writers latch onto that one scene where he helps her make a choice and just extrapolate it into a whole emotional thawing arc. You get a lot of stories where he notices her robotic compliance and decides, nope, we're fixing that, we're bringing out the real Kanao. It's basically comfort fic with a side of training montages.
They're often set after the final battle, exploring a quieter life the series never gave them. I've seen so many where he's helping her plant wisteria or something mundane, and she slowly learns what she likes outside of orders. The tropes are pretty gentle: shared trauma bonding, quiet domestic moments, maybe a festival date where she experiences something purely for fun for the first time. There's rarely any major conflict—the appeal is the healing and the softness, which, honestly, after the bloodbath of the manga, I'm all for.
Sometimes the dynamic flips, and you get fics focusing on her being freakishly strong and him being in awe of it, which is fun. But mostly it's just this very sweet, predictable, and honestly soothing exploration of two good people figuring out a peaceful existence together. It's my go-to when I need a break from angstier ships.