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.
Maybe there are some rebellious students as main characters. They are the ones who first realize that the situation in the school is like slavery and start to fight against it. They might be joined by other students who also feel the same way, and together they try to change the school environment that has been making them feel like slaves.
It could be a group of students who are being oppressed by a strict teacher or a clique of mean students. The students would be the main characters as they are the ones suffering from the'slave - like' conditions in the school. For example, the strict teacher might give them an excessive amount of homework and not allow any flexibility, making the students feel trapped.
In slave school stories, the central characters are the enslaved individuals. Some might be children with an eagerness to learn in the face of great danger. There could also be the slave owners, who play the role of the oppressors trying to prevent the slaves from getting educated. And then, there are those brave souls, like abolitionists or underground educators, who risked their lives to help the slaves gain knowledge.
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.
Typically, you'll see the main characters from the original High School DxD series like Issei Hyoudou. He might be in a different role in the fanfic, perhaps more involved in the'slave' aspects of the story. Rias Gremory could also be a main character, maybe as a figure of power who has'slaves' under her command.
As I don't know the exact 'Milliard Slave Story', I can't tell you the main characters. It could be the slaves themselves, perhaps a brave leader among them. Or it might include some of the slave owners or overseers who play significant roles in the story.
As I don't know the story, I can't tell for sure. But likely, the slave girl herself is a main character.
The main characters are the slave and the lion.
As I don't know the details of this 'full time slave story', I can't say for sure who the main characters are. It could be the slaves themselves, their masters, or perhaps other people related to the slave system like overseers or abolitionists.
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 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.