The RWBY team - Ruby, Weiss, Blake, and Yang - could also be important characters in the 'rwby four maidens fanfic'. They might interact with the maidens, either as allies or in some cases, having to resolve conflicts. For example, if there is a misunderstanding about the maidens' powers or intentions, the RWBY team would be there to figure things out.
There could be other original characters created specifically for the fanfic. These characters might be related to the maidens in some way, like a mentor to one of the maidens or a character who has a vendetta against the maidens and their powers. Their presence can add more depth and complexity to the story.
The knight would obviously be one of the main characters. He could be a new character created specifically for the fanfic, or perhaps a reimagined version of an existing RWBY character with knight - like qualities. The maidens would also be main characters. They could be based on the RWBY girls, or entirely new characters with unique abilities and personalities.
Jaune is probably the protagonist around whom the story revolves. As for the Maidens, they are likely characters with special powers. They could be a group of female characters with different personalities and abilities related to elements like fire, water, earth and air. Their interactions with Jaune would be a major part of the story, as they work together or face conflicts within the story's universe.
Well, one interesting plot could be the relationships between the four maidens and how they interact with the main RWBY characters. Maybe they team up to fight a common enemy.
Not having read 'the maidens a novel', it's difficult to determine the main characters. But it's reasonable to think that the maidens themselves are the central focus. There could be different types of maidens, for example, a bold and adventurous maiden, a shy and reserved one, etc. And they may interact with characters such as a wise old woman who gives them advice, or a young man who causes some sort of conflict or romance in their lives.
The knight could be developed by showing his code of honor. For example, in every battle he faces, he adheres to a strict set of rules. His backstory might be gradually revealed, like he was once a soldier in a great war and now he feels a sense of duty to protect the maidens. The maidens, on the other hand, could be developed through their interactions with the knight. Maybe one maiden is initially shy but as she spends more time with the knight, she becomes more confident in her abilities.
A major theme is the balance between good and evil. The Maidens' powers can be used for different purposes, and fanfics show them either being tempted by evil or firmly on the side of good. Legacy is also common. How the Maidens pass on their powers and what kind of mark they leave on the RWBY world is often explored in these fanfics.
Since I haven't read it, I can't say for sure. But I assume the 108 maidens are the main characters, and there might be some other supporting characters like wise elders or powerful adversaries.
Since I'm not familiar with the 'the dark maidens light novel', I can't tell you exactly who the main characters are. But typically in a light novel, the main characters are the ones around whom the story revolves. So in this case, it could be the 'dark maidens' themselves, and there might also be some supporting characters like friends, family members or foes related to them.
Obviously, Artyom would be one of the main characters. And then there would likely be the main RWBY characters like Ruby, Weiss, Blake, and Yang. They might interact with Artyom in different ways. For instance, Ruby could be curious about Artyom's weapons or his background.
It's also possible that the fanfic could focus on side characters from RWBY but in a new light related to the'shine' concept. For example, Penny could be a central character in the 'rwby shine fanfic' if the story is about how her robotic nature can shine in a different way, like her circuits starting to glow with a new kind of energy that changes her abilities and her role in the RWBY universe.