Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
I'm not sure exactly which 'ada learns dazai past fanfic' are out there. You could try searching on fanfiction websites like Archive of Our Own. They usually have a wide variety of fanfics on different topics and characters.
Well, first of all, the character of Dazai. His past is often filled with mystery and pain, so in a 'ada learns dazai past fanfic', the details of his past need to be engaging. This could include his relationships with other characters from his past, his motives, and his internal struggles. Then there's Ada. Her perspective and how she processes Dazai's past is significant. Does she empathize? Does she judge? Her actions and thoughts in response to what she learns are key elements. Additionally, the setting of the story, whether it's in a familiar place from the original work or a new location created by the author, can also play a role in enhancing the overall story of Ada learning about Dazai's past.
In a Giyuu x Shinobu fanfic, the setting and plot can vary widely. Some might be set during the main events of the Demon Slayer universe, with them facing demons together. Others could be more of an alternate universe story, where they meet under different circumstances, like in a modern - day high school AU. The fanfic could also delve into their backstories and how those experiences shape their relationship. For instance, Shinobu's past with her sister could play a role in how she opens up to Giyuu, and vice versa for Giyuu's own traumas.
I would recommend 'Whispers of the Heart' for Giyuu x Shinobu fans. This fanfic focuses on their unspoken understanding. It's full of small moments that build up their relationship, like when Shinobu leaves a small flower on Giyuu's doorstep as a sign of her care. Also, 'Shadows and Sunlight' is worth checking out. It has a great plot that involves them having to go on a secret mission together and how that journey changes their relationship from colleagues to something more intimate.