Sasuke could also be a main character. He may play a role in either helping or hindering Naruto's journey in this new side. Their relationship might be explored in a new light, with new challenges and reconciliations.
I don't know for sure, but probably the boy himself is a main character. There might also be some people he meets on 'the other side' who are important.
The main characters are the girl and the being from the other side.
It's also possible that there are characters from the original work (if this is a fanfiction based on something) that make appearances. They could play important roles in Naryo's journey, either by directly assisting or by creating situations that Naryo has to deal with in order to get to the other side.
The main characters in 'the other side miraculous ladybug fanfiction' are probably Ladybug and Cat Noir as they are the main protagonists in the original series. But it could also feature characters like Alya or Nino in different roles. Maybe Alya has a Miraculous of her own and is a key part of the 'other side' story. Nino could also be involved in some new and unexpected way, like being part of a resistance group against an evil force on this 'other side'.
Most likely, Harry Potter would be one of the main characters. Since it's based on the Harry Potter series, he would probably be central to the story, perhaps leading the fight against the evils of this 'hell' side.
I'm not sure specifically as I haven't read the novel in detail, but typically there would be a protagonist who is likely at the center of the story. Maybe someone who is on a quest to uncover the mysteries on the other side of darkness.
It could be members of the Akatsuki. Despite being antagonists in the original series, in the fanfiction they might have a change of heart. Maybe Pain, after seeing Naruto's resolve, decides to become his friend. Or it could be Orochimaru's former subordinates who see a different path through Naruto.
Some fanfictions might also introduce new or less - known Jinchuuriki characters. They could be from different hidden villages or have special backstories. These characters add more variety to the story and can bring new dynamics to the relationships between Naruto and the other Jinchuuriki. Their inclusion can also expand on the lore of the Jinchuuriki in the Naruto universe.
Sakura could be a main character. She might be the one who helps Naruto deal with his emotional and physical wounds after the war. She could also be struggling with her own feelings towards Naruto now that he's back.
I'm not sure specifically as I haven't read that particular 'Naruto' fanfiction. But generally, in 'Naruto' fanfictions, they could be about exploring different storylines, relationships, or powers that weren't fully explored in the original manga or anime. It might focus on a different perspective within the 'Naruto' universe, perhaps from the view of a side character or an antagonist.