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.
In 'the martian full story', the most important character is the astronaut left alone on Mars. His name might be Mark Watney in the movie adaptation. On Earth, there are scientists, engineers, and NASA officials who also become significant characters as they work towards rescuing him. Their combined efforts and the lone astronaut's struggle for survival form the backbone of the story.
I'm not entirely sure without reading it specifically, but likely there's an explorer or a scientist as a main character.
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.
There is usually a lead character, often an adventurous type, who is brave enough to take on the journey to Mars. There might also be some supporting characters, perhaps other members of the exploration team, who play roles in different parts of the story, like helping with scientific research or dealing with unexpected situations on Mars.
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.
The main plot is about an astronaut who gets left behind on Mars and has to use his knowledge and skills to survive while waiting for a possible rescue.
The main characters are the adoptive father and the boy who believes he is from Mars.
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 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.
One of the main characters could be a scientist. When the martian boy falls to earth, the scientist might be the first to discover him. The scientist would be curious about the boy's physiology and technology from Mars. Another character could be a young girl. She could befriend the martian boy and teach him about human emotions and relationships, which are very different from what he has on Mars.
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.