One great Akuma no Riddle crossover fanfiction is 'Akuma no Riddle Meets Another School'. It combines the unique characters from Akuma no Riddle with a different school setting, creating interesting new dynamics. The interactions between the assassins and the new characters are really engaging.
Another place is FanFiction.net. There are some really good and unique 'Akuma no Riddle' fanfics there. Just use the search bar and type in 'Akuma no Riddle' to find relevant stories.
One great 'Akuma no Riddle Fanfiction' is 'The Unspoken Bond'. It delves deep into the complex relationships between the characters. The author does an amazing job of exploring the emotional depth that the original series only hinted at.
The characters in Akuma no Riddle are very distinct. Each assassin has her own backstory, motives, and personality. When these characters are put into a crossover situation, it allows for a lot of exploration. Writers can develop new relationships between the Akuma no Riddle characters and those from the other crossover world, which is very appealing to readers.
Since I haven't read this particular 'akuma no riddle banba fanfic', it's hard to say for certain. However, generally in fanfics for 'Akuma no Riddle', they often explore the relationships between the characters in more depth. Maybe this one with 'Banba' focuses on his backstory, or his relationships with other characters in a way that the original work didn't fully explore.
To develop Banba's character, focus on her past experiences that made her the way she is. For example, if she had a difficult upbringing, it could explain her tough exterior. For Hanabusa, explore his dreams and ambitions. This can give depth to his character.
Tokaku is often portrayed as strong and protective. Her assassin - like skills make her a formidable character. But in fanfiction, her softer side for Haru is also a great trait. She is loyal and will do anything to keep Haru safe.
Their relationship could be full of tension at first. Isuke might be more reserved and Haruki more outgoing, creating an interesting dynamic. As the story progresses, they might start to complement each other. For example, Isuke's calmness could balance Haruki's impulsiveness.
The fanfic might focus on the unique dynamics between Tokaku and Haru. For example, it could show how their personalities, one being more reserved perhaps (Tokaku) and the other more open (Haru), complement each other. It may also explore their shared experiences and how those shape their relationship within the story.
There could also be an antagonist or a group of antagonists. They would be the ones creating obstacles for the main character and other protagonists. Their actions and motives would be crucial to the development of the mystery and the story as a whole. They might be trying to prevent the riddle from being solved for their own gain or to maintain some sort of power or control within the world of the light novel.