It's difficult to say exactly without having read the whole collection. However, we can assume that there could be scientists among the main characters. They would be interested in studying the Martian environment and the monster. There might also be some sort of military or security personnel if the situation with the monster gets out of control.
One of the main themes could be survival. In stories about the Martian monster, characters might have to fight for their survival against this unknown and dangerous entity. Another theme could be exploration, as the setting is often on Mars, which implies the human drive to explore new frontiers.
Since it's 'The Monster and Other Stories', there may be a group of adventurers as main characters. They might be on a quest that leads them to encounter the monster. Each of them could have different personalities and skills. For instance, one could be strong and fearless, while another is intelligent and good at solving puzzles. And of course, the monster itself could also be considered a main character in a way, as it drives the action and conflict in the stories.
The one about the Martian Monster is surely interesting. It might have unique characteristics and behaviors that make it stand out among the stories.
Often, the main character is still Mark Watney, the astronaut who was stranded on Mars in the original story. He continues to be the central figure as the fanfiction builds on his experiences.
In 'The Monster', Henry Johnson is a main character. He is the one who suffers a disfiguring accident.
I don't know specifically as it's not a well - known work with established characters. But they could be things like a fearless Martian explorer, a wily Martian native, or a devious Earthling who has come to Mars for his own gain.
The main character is the astronaut who is left on Mars. I'm not sure if there are other really prominent characters without looking more closely at the story.
It might also be that the main characters are the spacecraft themselves. For instance, the Mars rovers like Curiosity or Perseverance. These rovers are like the protagonists in the exploration of Mars. They move around the Martian surface, take pictures, analyze samples and send back valuable information. They are designed and built by teams of engineers and scientists, but in a way, they are the ones directly experiencing the Martian environment and are central to the story of Martian exploration.
The key character is Mark Watney. He's a botanist - astronaut. Then there are the NASA team members back on Earth. They include Venkat Kapoor, who is involved in the decision - making process regarding the rescue mission. Mitch Henderson, who has a role in the operations at NASA. And there are other scientists and engineers at NASA who all contribute to the efforts to save Watney from Mars.
It might be more focused on Martian - related stories. While other collections could be more diverse in terms of settings, 'Martian Monster and Other Stories' seems to have a central theme around Mars and its associated elements.