I'm not sure specifically as I don't have the full story, but it could be some explorers. They are usually the ones who would venture into a cave in a short story like this.
I don't know the story, so I can't tell who the main characters are.
The main characters are the group of prisoners in the cave. They are in a state of ignorance, believing the shadows on the wall to be real. Then there is the special prisoner who is released. He experiences a transformation as he discovers the truth about the real objects that create the shadows and the outside world. There may also be those who are outside the cave in the true world, though they are not as much in focus as the prisoners.
As the story isn't known to me, the main characters might be a group of friends who decide to explore the dark cave out of curiosity. Or it could be a solitary adventurer seeking solitude or a particular treasure hidden within the cave.
There could be an explorer as a main character, who is the one to enter the cave and face the monster.
In some 'cave man stories', there could be a hero - like a cave man who saved the group from a big predator. He became a symbol of courage for the whole community. And there might be the shaman - like figure who was believed to have some special connection with the spiritual world and could perform certain rituals for the well - being of the group.
There could be an explorer. They are often the ones who dare to enter the cave and face the monster.
It might be a group of friends. You know, they hear about the spooky cave and decide to check it out for fun or out of curiosity. They could be young and naive, not fully realizing the danger they are getting into, and then they have to face all kinds of scary things in the cave together.
Perhaps the main character is a curious individual, like a child or a young adult. And there might be some animal characters as well that he encounters during his adventure in the cave.
I don't know specifically as we don't have much information about this 'Balrog Cave Story'. But it could be a hero who dares to enter the cave where the balrog lives.
I'm not sure specifically without more information about the story. It could be a group of explorers, perhaps some young and adventurous friends.