The main characters would be Percy and Calypso. However, depending on the direction of the fanfiction, some minor characters like the spirits or minor deities associated with Ogygia could also play significant roles. For instance, if the story involves Percy learning about the history of the island, those minor characters could be the ones to tell him the stories and thus become important in the narrative.
Well, it could be about Percy, perhaps from the Percy Jackson series, staying on Ogygia. Maybe it explores his experiences on that island, like his interactions with the nymph Calypso who lives there. It might also touch on how he copes with being stranded or what new things he discovers about himself and the place.
In 'Percy Stays Behind' fanfiction, Percy is the main character. But in addition to that, there might be characters from the periphery of the original story that now come into focus. For instance, if there were some minor characters in the original who were local to the area where Percy stays behind, they could become more prominent. They might have their own stories and motives that interact with Percy's situation. Also, any magical or supernatural beings specific to the setting of the fanfiction could be important characters as well.
Another character that could be involved is Nico di Angelo. Since he has some experience with the Underworld and Tartarus, he might try to find a way to communicate with Percy or even attempt a rescue mission. His powers and knowledge would play an important role in any interaction with Percy in Tartarus.
He could also meet some minor nymphs or spirits that inhabit the island. These beings might be curious about Percy's return and have their own stories to tell. For instance, there could be a water nymph who has some knowledge about a prophecy related to Percy and his return to Ogygia. Or a spirit that has been observing the changes in Ogygia since Percy last left and can give him some insights on what has been going on in his absence.
Perhaps a Roman god who has been forgotten or feels slighted. This god could be causing chaos in the Roman camp, and Percy, as a demigod staying Roman, has to figure out how to appease or defeat this god. Since Roman gods can be very temperamental, it would be a difficult task for Percy.
Well, clearly the main characters are May and Ash. May is a Pokémon coordinator, and Ash is a Pokémon trainer. Their different skills and personalities make their interactions in the fanfiction really interesting.
Well, Percy could bring some modern - day items with him that might amuse or baffle the inhabitants of Ogygia. He might introduce Calypso to things like a smartphone (if it works there) and show her pictures of his friends and family. This could create an interesting cultural exchange between Percy's world and the world of Ogygia.
The main characters would be Hermione and the Malfoys. Hermione, being the intelligent and strong - willed Gryffindor. Among the Malfoys, Draco would be at the forefront. He has his own insecurities and growth arc that can be intertwined with Hermione's stay. Narcissa could act as a mediator of sorts between Hermione and the more hostile members of the family like Lucius, who might initially see Hermione as a threat to their pure - blood ideals.
Obviously, Ron and Hermione are the main characters. Ron, with his red hair and freckles, is the loyal friend who decides to stay by Hermione's side. And Hermione, the brilliant witch with her bushy hair and vast knowledge, is the one he's staying with.
Draco would surely interact with the Dursleys. Vernon would probably be very suspicious and hostile towards Draco at first, seeing him as another strange and magical being. Petunia might be a bit more reserved but also curious in her own way. Dudley could range from being intimidated by Draco's wizarding background to being fascinated. And don't forget about Hedwig! If she's still around, Draco might have some sort of interaction with the owl, which could be really funny or interesting as he tries to figure out how to deal with this magical creature in the Muggle - dominated Dursleys' household.