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.
It could be an old and wise slave. He has seen many things in his life and uses his experience to advise the king. He is respected by some in the court for his wisdom but also feared by others because of his influence on the king. His story would be about how he manages to keep his position and help the king rule better despite being a slave.
Most probably, the two main characters are the alpha king, who holds a position of great power and authority, and the slave who he hates. There might be other characters too, like the king's advisors or other slaves, but the focus seems to be on these two.
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.
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.
There are different characters in 'For the King'. You can have a warrior - type character who is strong in combat. There is also often a mage - like character who can use magic to fight enemies or solve puzzles.
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.