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.
Another character that could be important in this fanfiction is Remus Lupin. He has a connection to both Sirius and Harry. He might play a crucial role in the escape plan, perhaps using his knowledge of the wizarding world and his friendship with Sirius to aid in the escape. Also, Dumbledore could be a significant presence, guiding Harry and his friends from the sidelines, providing cryptic advice that helps them in their mission.
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.
In some fanfics, Harry Potter might escape from Azkaban through powerful magic spells that he manages to conjure secretly. Maybe he uses a spell to create a diversion among the Dementors and then slips away while they're distracted.
In some fanfics, he might use powerful magic that he has secretly mastered. Maybe a new spell that can create a portal right out of his cell.
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.
In some fanfictions, a plot twist could be that the Ministry of Magic has been secretly working with Voldemort's followers to keep Sirius in Azkaban. And Harry, Ron, and Hermione find out about this conspiracy. They then have to not only break Sirius out but also expose the Ministry's dark secrets. This would add a whole new layer of political intrigue and danger to the story.
Obviously, Harry Potter himself is a key character as the story is about his release from Azkaban. Dumbledore is likely to be important too. He has the power and influence within the wizarding world to potentially free Harry. His wisdom and knowledge of the situation would play a big role.
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.
Harry Potter himself is obviously a main character. He's usually the one trying to escape. His friends Hermione Granger and Ron Weasley are often involved too, as they would never leave Harry alone in such a situation.