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.
One of the main themes could be mystery. The 'riddle' in the title likely implies a lot of mystery elements throughout the story.
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.
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.
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.
Koko Hekmatyar is a very notable character. She is a young and brilliant arms dealer. She has a charming yet dangerous personality. She's able to navigate the complex world of arms trading with ease and is highly intelligent, always one step ahead of her opponents.
Since I haven't read it, I can't say for sure. But based on the title, the main characters are likely to be the 'oroka na tenshi' (the naive angel) and the 'akuma' (the devil).
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.
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.
Then there's Alexis Douse, who is part of the ruling class and his actions and motives are crucial in understanding the political side of the Grancrest War. His character is complex as he has to balance his personal ambitions with the needs of his faction in the context of the ongoing war.