The main characters include Sans, who is much more menacing in Underfell. Papyrus is also a main character, but with a more villainous streak compared to the normal Undertale Papyrus. And of course, the human who falls into the Underground is a key character as the story follows their journey through this dangerous world.
The main characters in the Underfell original story are the ones that make this darker version of the Undertale world come alive. First, there's the human, who is thrust into this strange and hostile Underground. Sans is a major character, with his menacing presence and a different moral compass. Papyrus, in this story, is a far cry from his original, friendly self. He's more of an antagonist. And we can't forget Flowey, whose already complex nature in Undertale is made even more so in Underfell, being more devious and unpredictable in his actions towards the human.
In the Underfell original story, it's a darker take on the Undertale universe. The characters are more aggressive and the world is harsher. For example, Sans is more menacing here. The story still involves a human falling into the Underground, but they face more dangerous and morally grey characters compared to the original Undertale.
Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.
If it's a different '300 original story', perhaps the main characters are the leaders of the 300. For example, if it's a group on a quest, the ones who are in charge of leading the 300, making decisions, and guiding them through the various trials and tribulations would be the main characters.
In the original 'Alice's Adventures in Wonderland', the main characters are Alice, the White Rabbit, the Cheshire Cat, the Mad Hatter, and the Queen of Hearts.
Sans is often a very popular character in these fanfictions. In both Undertale and Underfell, he has a distinct personality. In Underfell, his more edgy and somewhat darker side can create interesting interactions when combined with the regular Undertale Sans.
In the original batman story, there are several main characters. Bruce Wayne, who is Batman, is of course central. He is the one on a mission to save Gotham. Alfred, his butler, is a very important character as he provides support, both emotional and practical, to Bruce. Commissioner Gordon is also a main character. He is an ally of Batman in the fight against crime in Gotham. He helps Batman in various ways, like sharing information about crimes in the city.
The main character is Charles Lee Ray, the serial killer whose soul gets transferred into the doll, and of course, Chucky the doll itself.
The main characters are a priest and a novitiate. They are the ones sent to the abbey to deal with the situation regarding the possessed nun.
The main characters in the 'Pitcairn Original Story' are likely the Bounty mutineers. They were the ones who initially settled on Pitcairn Island.
Since I'm not sure which specific 'cabaret original story' you mean, it could be the performers. For example, singers, dancers, and comedians who were the heart of the cabaret shows.