The sand kings are surely main characters. There might also be other characters like humans or other creatures that either interact with the sand kings, try to study them, or are in some sort of conflict with them in the 'Sand Kings short story'. But the sand kings should be the focus.
Most likely the sand kings themselves are the main characters. Since the story is named after them, they would play a central role.
One main feature is their connection to sand. They can manipulate it in different ways. Another is their likely residence in desert - like areas. And they are usually depicted as quite powerful.
Well, generally speaking, the 'Sand Kings short story' could be about the sand kings' life. It may include their interaction with the environment, perhaps a battle for territory or resources. There could also be a description of their physical features which are adapted to their sandy habitat. Maybe there are other characters in the story that interact with the sand kings in different ways.
Probably the most prominent would be the leading Tomb King. He would be in charge of leading his undead forces, making decisions about battles and the defense of their tombs. There might also be necromancers who serve the Tomb Kings, playing important roles in raising more undead and maintaining the magic that keeps the Tomb Kings' power intact.
Since I don't know the exact story, I can only guess. They could be three powerful kings, perhaps named something like King Arthur, King Henry, and King Richard.
The narrator is a key character. He is the one who comes across the strange book and becomes involved with it.
The main character is the narrator. He is the one who comes into possession of the Book of Sand and is deeply affected by it. His thoughts and reactions drive the story forward.
In the 'Sand Man short story', the most prominent character is surely the Sand Man. However, there might also be children who are the recipients of his sand - sprinkling and sleep - bringing actions. These children can be considered important characters as well, as the story often revolves around their interaction with the Sand Man, whether it's a fearful anticipation of his visit or the experience of the dreams he might influence.
One of the main characters is Kaladin. He is a complex character with a lot of depth.
Since we don't have the actual story, it's impossible to know for sure. But typically, there would be a 'king' character who is in a position of power within the game. There might also be other players who are either friends or foes, depending on how the story unfolds.
Given the title 'two kings short story', the two kings are probably the central characters. But in a short story, there could be other characters that are crucial to the development of the plot. Maybe there is a young hero who serves one of the kings and through his actions, the relationship between the two kings is affected. Or perhaps there is a wise old woman who gives advice to one of the kings, which then influences the actions of both kings. It all depends on the specific details of the story which unfortunately I'm not aware of.