Villains play a major part too. The Riddler might be eager to use this new information to create puzzles and challenges specifically for Bruce Wayne. Catwoman could also be involved, perhaps her relationship with Batman takes a new turn as she has to decide how to react to his public identity, whether to help protect him or take advantage of the situation.
Batman's allies like Robin (whichever version - Dick Grayson, Jason Todd etc.) are often involved. They might have to deal with the fact that their mentor's identity is now public and face new threats because of it. Also, Commissioner Gordon is a main character. His relationship with Batman has always been complex, and this revelation will surely change how they interact.
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.
Well, obviously Batman himself is the main character. He's the one who gets caught up in the Yellow Lantern concept. Then there are often the Yellow Lantern Corps members. Characters like Sinestro might make an appearance as he is a major figure in the Yellow Lantern Corps. They would be the ones trying to influence Batman or fight against him depending on the plot.
In many cases, Ra's al Ghul, who is often associated with the Demon Head concept, is a central figure. His complex relationship with Batman adds a lot of depth to the fanfiction. Also, characters like Alfred, Batman's loyal butler, may play important roles. He could be the one providing Batman with crucial information or moral support during the encounters with the Demon Head. There may also be other members of the Bat - family, such as Batgirl, who might get involved in the story to help Batman deal with the Demon Head - related threats.
Hermione is often involved. She might be the one helping Harry with the spells or potions to change his identity, like brewing the Polyjuice Potion correctly. Ron could also be involved, perhaps providing moral support or getting into trouble while trying to keep Harry's new identity a secret.
In addition to Danny and his immediate circle, there could be other characters from the Danny Phantom universe. For example, Valerie Gray. If she discovers Danny's identity, she might have a complex reaction. On one hand, she's been hunting ghosts, but on the other, Danny has also been protecting the town. So, her role in the fanfiction could be very interesting, especially in terms of how she interacts with Danny after the identity reveal.
Obviously, Ladybug and Chat Noir are often central. Marinette, who is Ladybug, and Adrien, who is Chat Noir. Also, Hawk Moth as the main villain often plays a role in these fanfictions when the identity is about to be revealed or has been revealed, as his actions can be related to this big event.
Obviously, Batman/Bruce Wayne himself is at the center. Alfred Pennyworth would be very involved as he has known Batman's identity all along and would likely be trying to protect it or deal with the consequences if it's revealed. Commissioner Gordon might also play a big role. He has a close relationship with Batman and would be affected by the revelation. Then there are the villains. The Joker could be one of the main villains in such a story, using the revelation to his advantage in some devious way.
One interesting plotline could be the reaction of Gotham's criminals. They might either be overconfident thinking they can now easily target Batman or be terrified knowing his true identity means he has more resources at his disposal. Another could be how Batman's allies, like Alfred, deal with the new situation. Alfred might be even more protective or start to worry about the safety of Bruce Wayne's loved ones.
Kakashi could be involved too. Given that Kakashi has trained Naruto and seen his growth, Naruto might trust him with his secrets. He could reveal his thoughts on how he wants to change the ninja world for the better and the steps he is planning to take in the future.
Obviously, Batman/Bruce Wayne himself is a central character. His life would be turned upside down by the revelation of his identity.