One key element could be his origin as a human. His human experiences surely had an impact on his demon self. Another might be any significant relationships he had before becoming a demon.
Upper Moon 4's back story could involve a tragic event in his human existence. Maybe he was betrayed by someone he trusted deeply. This betrayal could have been so severe that it pushed him over the edge, and when he was turned into a demon, his new nature was shaped by that past hurt and anger. His actions as an upper - moon demon are likely influenced by the unresolved emotions from his human days.
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.
Some Avengers might be suspicious. For instance, Captain America, who values order and loyalty, might be cautious about Moon Knight's erratic behavior. Iron Man, on the other hand, could be intrigued by Moon Knight's technology, especially if it has any connection to his lunar - based powers. Thor might see him as an interesting mortal with a connection to a different kind of divinity compared to his Asgardian gods.
Some characters might laugh. If the fanfiction has humorous elements, like a comical take on their battles or relationships, they would find it entertaining. For instance, if there's a fanfiction where the Sailor Scouts accidentally swap powers and get into all sorts of wacky situations, the characters themselves might chuckle at the absurdity.