Perhaps there is a fellow slave or a sympathetic free person in the novel. This character could play an important role in the slave girl's journey, either by providing support, friendship, or a different perspective on her situation.
As I don't know the story, I can't tell for sure. But likely, the slave girl herself is a main character.
Since I haven't read 'slave to the darkness novel', I can only speculate. There could be a protagonist who is forced into this state of being a slave to darkness. Maybe there is a mentor - like figure who tries to help the main character break free, or there could be other characters who are also trapped in the same situation and they bond over their shared plight.
Since I haven't read the 'slave to darkness novel', it's hard to say for sure. However, the main characters might include a protagonist who is on a path to self - discovery while being a slave to darkness. There could also be an antagonist representing the pure essence of the dark power that controls the slaves. And perhaps there are other supporting characters who are either fellow slaves or those trying to free the slaves from the clutches of darkness.
Obviously, the boy slave himself is one of the main characters. He is the central figure around whom the story revolves. There may also be the slave master, who represents the oppressive force in the boy's life.
The black knights, of course. Their actions and decisions greatly influence the story. They are in a position of power and their relationships with the slaves are central to the plot.
Since I haven't read this particular web novel, it's hard to say exactly. However, based on the title, the main character is likely the shadow slave. There could be other supporting characters like fellow slaves who are in the same situation, or maybe a mentor - like figure who helps the shadow slave discover their potential. There could also be characters from the upper echelons of the society in the story's world, who either oppress or are intrigued by the shadow slave.
One main theme could be oppression. In the 'the slave girl novel', it might show how the slave girl endures various forms of oppression, like being deprived of basic rights and freedoms. Another theme could be survival. She has to fight against the harsh conditions to stay alive.
In slave boy stories, the central character is, of course, the slave boy. He experiences the brutality and unfairness of slavery first - hand. There might also be a slave master's son or daughter. Their relationship with the slave boy can be complex, ranging from friendship to further exploitation. Another main character could be a runaway slave who serves as an inspiration for the slave boy. This runaway slave has tasted freedom and might encourage the boy to strive for the same.
Since I don't know the exact'school slave story', it could be students who are the main characters. They might be the ones facing the difficult situations that make them feel like slaves.
There are the king, who is likely in a position of great luxury and control, and the slave, who is probably at the bottom of the social hierarchy. These two characters are central to the story as their relationship, whether it's one of conflict, friendship, or mutual growth, drives the plot forward.