His friends like Sam and Tucker are likely to be main characters too. They usually support Danny in his adventures and add a human element to the story. Sam with her gothic style and strong will, and Tucker with his tech - savvy skills are important parts of Danny's world.
There might also be some new characters introduced in the fanfiction that play a major role. Maybe a powerful ghost that is the source of the'suffering' in the title, who has a complex backstory and motives for causing trouble for Danny.
The main antagonist could be one of the powerful and malevolent entities from 'The Suffering'. For example, the big bad demon or the tortured soul that has been corrupted by the darkness of that world and now wants to spread it to Danny's world.
Obviously, the main character would be Danny Phantom himself, but in two different forms or versions. One would be the original Danny Phantom from the canon source, and the other would be a version created by the fanfiction. There might also be his friends like Sam and Tucker who could play important roles in helping the two Dannys understand each other and deal with any issues that arise from their meeting.
In addition to the ghosts, there might be some human characters that are important in the 'phantom city danny phantom fanfiction'. They could be Danny's friends or family members who get involved in the events of the Phantom City. Their reactions to the supernatural world and their interactions with Danny and the ghosts would add an interesting dynamic to the story.
One possible interesting plotline could be Danny facing a new and more powerful ghost that has a personal vendetta against him. This would test his limits and force him to discover new powers or ways to fight.
Obviously, Danny Phantom and Dan Phantom are the main characters. Danny is the original form, a heroic character, and Dan is his more evil and powerful transformation.
One idea could be to explore how Danny's ghost powers interact with the dark and tortured world of 'The Suffering'. Maybe he gets trapped in that nightmarish realm and has to use his unique abilities to survive and find a way out. Another idea is to have some of the tortured souls from 'The Suffering' cross over into Danny's world and how he deals with them while keeping his identity as a half - ghost a secret.
In many fanfictions, there could be new characters created specifically for the Renegades. These might be other half - ghosts or humans with special abilities that complement Danny's. For example, a character who can manipulate time in a small way to help the group in battles.
Jazz, Danny's sister, often plays an important role. She might use her psychological knowledge and her understanding of Danny to try to solve the mystery of his disappearance. And of course, some of Danny's ghostly foes might also be involved, as their actions could be related to his vanishing.
Well, clearly Danny Phantom is likely to be one of the main characters. And since it's about devils, there must be some devil characters as well. Maybe there's a lead devil character that Danny has to face off against.
Well, obviously the twins would be main characters. They could be named something like Jake and Jill. Danny himself would also be a main character as he would likely be involved in their story, either as a mentor or a friend. Then there could be other recurring characters from the original Danny Phantom series like Sam and Tucker, who might help the twins adjust to their new situation.