There's no way to know for certain without reading the fanfiction. However, it could be a new set of characters created by the fanfiction author. They might introduce characters who are specifically there to experience and deal with the consequences of the party. Maybe a mysterious stranger shows up at the party and is somehow involved in all the events that follow.
It could be about a party where actions at that party lead to various outcomes. Maybe there are characters who make choices at the party that later have big impacts on their relationships, like a character getting into an argument with a friend and then they have to deal with the consequences of a damaged friendship. Or perhaps someone makes a romantic move at the party and then has to face the resulting emotional fallout.
If it's a crossover with Final Fantasy, Cloud Strife could be a main character. He could bring his sword - fighting skills to the Mario Party world. His serious and brooding personality would contrast with the more light - hearted Mario characters, creating some interesting interactions during the story and mini - games.
Since I haven't read 'the after party novel', I have no idea about the main characters. But usually in a novel, the main characters are those who drive the story forward. They could be the party organizers, or some special guests who have their own stories and motives.
I don't know as I haven't read it. It could be a host of the party, a guest who stands out, or a group of friends who are all main characters in their own ways.
I don't know as I'm not familiar with this particular novel. But typically in a dinner party - themed novel, there could be the host or hostess as a main character.
In the Party Monster Story, the primary character is the Party Monster. It could be a flamboyant and energetic being. There may also be a group of friends or followers who are often around the Party Monster at these parties. These could be characters with different quirks and personalities, like a shy but curious one, a wild and crazy one who is always up for any party antics, and perhaps a more sensible character who is constantly trying to keep things in order but often fails in the face of the party's chaos.
There could be a host who organized the garden party. Maybe a wealthy lady or a friendly family.
I don't know the exact characters as I haven't read the book. But there would probably be the party girl herself as the main character. There might also be her friends who she parties with, and perhaps some love interests she meets at parties.
Jibanyan would likely be one of the main characters. He's very popular and loves to have fun, so he'd be right at home at a pool party. Whisper could also be there, providing some interesting facts about the pool or the yokai. And Komasan, being the friendly yokai he is, would surely be among the main characters enjoying the pool party.
One of the main characters was George Donner. He was sort of the leader of the group. There were also many other families and individuals who were part of the party. For example, the Reed family was also quite prominent among them.