Obviously, Harry Potter is the central character. He's the one who is wrongly imprisoned in Azkaban.
There could also be other characters like Dumbledore or Sirius Black. Dumbledore might be involved in the background, using his wisdom and influence to help Harry's case. Sirius Black, having his own experience with being wrongly accused and imprisoned, could play a role either in spirit (through memories or messages) or if he's somehow freed and able to assist Harry directly.
His friends like Hermione and Ron would most likely be main characters too. They would be the ones outside Azkaban, using their intelligence and courage to try and free Harry. Hermione, with her vast knowledge of magic and books, would be devising plans and Ron would be there for moral support and to add his own brand of wizarding skills to the rescue efforts.
Obviously, the child is one of the main characters. There might also be some prison guards who play important roles, either as antagonists or those who gradually change their views towards the child. And perhaps some other inmates who interact with the child could also be considered main characters.
There could also be new characters introduced specifically for this fanfiction. Maybe there's a vampire hunter who comes to Hogwarts thinking the vampire is a threat. This character would add a new layer of conflict and intrigue. Additionally, the teachers at Hogwarts, like Dumbledore, would still play significant roles as they try to maintain order in the school while dealing with this new and rather unusual situation.
Dumbledore might play an important role as well. He would be guiding Harry through this new and strange situation. He could have some secrets about the Lord of Azkaban that he reveals to Harry over time. Also, some of the Azkaban guards or prisoners could become significant characters as Harry tries to reshape Azkaban.
Obviously, Harry Potter himself would be a main character. He would be at the center of any story that involves 'Harry Potter rules Azkaban' as he is the one who is supposed to 'rule' in some way.
Obviously, Harry Potter himself is a main character. Hermione and Ron often play important roles too. They might try to break Harry out of Azkaban or help him from the outside. Sometimes, Sirius Black could also be involved, especially if the story is related to the secrets of Azkaban or the wrong imprisonments. Dumbledore could also be a key figure, guiding Harry or influencing the events from afar.
Obviously, Harry Potter is one of the main characters. He is likely to be the driving force behind the attempt to get Sirius Black out of Azkaban.
One possible plotline could be Harry's attempts to prove his innocence while in Azkaban. He might use his magical skills in unexpected ways to communicate with the outside world and gather evidence.
The Dementors could be the main antagonists. They are always a threat in Azkaban, sucking out the souls and happiness of the prisoners. They would be constantly trying to get at Harry and Daphne.
It could be the corrupt prison guards or wardens. They might be in league with dark forces outside Azkaban. They oppose Harry and Ginny because the couple is getting too close to uncovering their illegal activities within the prison, like smuggling dark artifacts or collaborating with Death Eaters.
Obviously, Harry Potter himself is the central character. Hermione and Ron would most likely be involved too. They would probably try to find a way to get Harry out of Azkaban. Dumbledore might also play a role, perhaps guiding Hermione and Ron from behind the scenes or even trying to influence the wizarding court to re - evaluate Harry's case.