The victim might be a main character in a sense. Their story, their life before the mystery event, can be an important part of the plot. Also, the suspect could be a main character as the whole story may revolve around proving or disproving their guilt.
A detective could be a main character. He or she is the one who tries to solve the mystery.
In mystery fiction, it's not just the obvious ones. Sometimes a journalist who is investigating the case can be a main character. They have their own ways of getting information and can bring a different perspective to the mystery. Or an eyewitness who saw something strange related to the mystery could also be a main character, as they hold crucial information that could either solve the mystery or lead everyone astray.
In Poe's works, there are also detectives or those with a curious nature. C. Auguste Dupin in 'The Murders in the Rue Morgue' is an early example of a detective - like character. He uses his intelligence and powers of observation to solve the mystery, which is quite different from the more typical, troubled main characters in other Poe stories.
I don't know for sure. It could be a group of young friends who live in Prides Crossing.
There are also characters like Tommy and Tuppence Beresford. They are a young couple who often find themselves involved in various mysteries. Their relationship and their different personalities contribute to the story as they work together to solve the cases in novels like 'The Secret Adversary'.
One of the main characters could be the detective - like figure. He is often intelligent and observant, always on the lookout for clues. For example, in some novels, he might be a private investigator who has a reputation for solving difficult cases.
In the 'Mystery of the Emblem Novel', there could be a young protagonist who stumbles upon the mystery of the emblem by accident. There might also be an old sage - type character who has some knowledge about the emblem's history but is reluctant to share it all at once. And perhaps there's a villain who wants to use the emblem for their own malicious purposes, like taking over a kingdom or something similar.
There could be a variety of characters. There might be a strong and resourceful man who is good at building shelters and finding food. There could also be a woman with a kind heart who takes care of the others emotionally, and perhaps a child who brings out the protective side of the other characters.
I don't know the specific main characters in 'the mystery guest novel' as there's not enough information given. But usually, there would be the mystery guest themselves, and perhaps a protagonist who tries to figure out the mystery around the guest.
There are also some recurring characters like the police captain who Columbo reports to. This character adds another layer to the story as he sometimes questions Columbo's unorthodox methods but in the end is always impressed by his results. And then there are the witnesses, who provide important clues for Columbo to solve the mystery.
I don't know the exact main characters as I haven't read it. But likely, the doctor is one of the main characters. There could also be a nurse or a patient who plays an important role in the story.
There could be a lot of potential main characters in a dime mystery novel. It might have an amateur sleuth who stumbles upon the mystery. There could also be a police officer who is initially skeptical of the sleuth's abilities but later teams up with them. And perhaps a victim's family member who is desperate for answers and helps in the investigation in their own way.