Obviously, Danny Phantom would be the main character. He would be the one bringing his unique set of powers to the Civil War setting. There might also be some key historical figures from the Civil War like Abraham Lincoln. Maybe Danny has an encounter with him and helps him in some way.
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 the 'Danny Phantom in the Civil War' fanfic, perhaps Danny is somehow transported to the time of the Civil War. He could find himself in a moral dilemma. On one hand, he wants to stop the bloodshed, but on the other, he doesn't want to interfere too much with history. He might end up using his powers in a more subtle way, like using his ecto - energy to heal the wounded without being noticed.
Obviously, Danny Phantom and Ember would be the main characters. Danny is the half - ghost protagonist who is being raised by Ember. Ember, a ghost with music - related powers, would play the role of his caretaker or mentor in this fanfiction.
Danny Phantom is for sure the main character on the saving side. On the Justice League side, all the classic members would probably be there. Superman, being the symbol of hope and having super strength, heat vision etc. Batman, the Dark Knight with his gadgets and detective skills. Wonder Woman with her Lasso of Truth and superhuman strength. The Flash, who can move at incredible speeds. Green Lantern, who can create constructs with his power ring. And Martian Manhunter, with his shape - shifting and telepathic abilities. They all could be in danger and Danny would have to figure out how to save each of them based on their unique powers and vulnerabilities.
Obviously, Danny Phantom himself. And his classmates, which could include Sam and Tucker. They are Danny's closest friends and are likely to play important roles in the story.
Obviously, Danny Phantom is one of the main characters as the title implies he's the one doing the pranking. And then, members of the Justice League like Superman, Batman, Wonder Woman, Flash, etc. would be the ones being pranked.
In a crossover, characters from the other franchise might betray Danny. For example, if it's a crossover with a show about vampires, a vampire character who seems friendly at first could betray Danny for his ghost powers. They might want to use his powers for their own dark purposes. Another likely candidate could be a character who is jealous of Danny's abilities. Maybe in a crossover with a superhero world, a sidekick who feels overshadowed by Danny's powers and popularity might be the one to betray him.
Danny is surely a main character. Then there are his family members. His sister Jazz, for example, could be a key character in the fanfiction. She's smart and could get involved in Danny's ghost - related adventures in different ways. Also, some new characters might be introduced. Maybe a mysterious ghost hunter who at first seems like a threat but later turns out to be an ally. But Danny will always be at the center of the story, with his dual identity as a high school student and a ghost - fighting hero.
Well, apart from Danny, Sam and Tucker, there could be other characters from the Danny Phantom universe. For example, Valerie Gray, who has her own vendetta against ghosts. When Danny reveals his identity, she might have a really interesting reaction. She could either see him as a threat or realize that he's not as bad as she thought. And then there are the teachers at school. They might notice some changes in Danny after his identity is revealed and start to suspect something strange is going on.
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.