The students at Hogwarts would have a range of reactions. The younger students would be very frightened and might huddle together in the common rooms. Some of the braver Gryffindors, like Neville, would try to stand up to Laughing Jack, even if they were shaking in their boots. The Hufflepuffs would probably try to offer help or kindness, thinking that Laughing Jack might be a misunderstood being. The Ravenclaws would be analyzing every move that Laughing Jack makes, trying to figure out his nature and how to deal with him. And the Slytherins would be divided, some seeing an opportunity to gain power if they could control Laughing Jack, while others would be cautious and stay out of his way.
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.
A cold smile or a hot haha was a kind of opposite smile. A cold smile was a smile that contained contempt, ridicule, helplessness, anger, and so on, while a hot haha was a kind of passionate smile. The cold smile and warm laughter were fake and pretentious, making people's hair stand on end. The word 'narrow-minded' was used to describe a person who was careless and big-hearted. Sometimes, it also implied that he was simple-minded. This word was usually used to describe a person's attitude of letting things that were related to their own interests develop in a bad direction.
One common theme is the exploration of their relationship. Given their different personalities in the original work, fanfictions often focus on how they bridge the gap between them, like how Giyuu's stoicism and Shinobu's more cheerful yet calculating nature interact.