Another important character could be an old Azkaban warden who has been there for ages and knows all the secrets of the prison. He could initially be resistant to Harry's new ideas but later become a mentor. Also, some of the prisoners might have important roles. For example, a wrongly - accused wizard who has unique knowledge about a hidden power within Azkaban that Harry needs to uncover to fully inherit and transform the place.
Obviously, Harry Potter himself is the most important. Then there could be some old villagers who know all the secrets of the village and guide Harry.
Well, obviously Harry himself is the central character. But aside from his friends, characters like Draco Malfoy could have a different relationship with Harry. Instead of being just a rival, he could be an ally of sorts in Harry's dark journey, or perhaps he tries to use Harry's newfound darkness for his own gain. Snape too could be important. Given his complex relationship with Harry, he might be in a position where he has to choose between helping Harry return to the light or letting him continue down the dark path.
Perhaps in this fanfiction, Harry inherits Azkaban due to some dark family secret. He then has to deal with the dark forces that have been lurking within the prison walls for centuries. He might find a way to use the prison's own dark magic against Voldemort's remaining followers who are hiding in the shadows.
Hermione would probably be involved. She could help Harry research about the island, like its history and any magical properties it might have.
Harry Potter himself, of course. He has always been at the center of things in the wizarding world. His actions and decisions would likely have a great impact on any change that occurs.
Letters often serve as a means of communication between characters in different locations. In fanfiction, they can carry important plot - driving information. For example, a letter from Dumbledore could reveal a hidden prophecy that sets the course of the story.
Neville could play an interesting role. Maybe he could be the one who discovers a new magical plant that is needed for a potion to cure an illness that Ginny or Harry contracts during their marriage. Luna could also be involved. She might come to them with some strange but useful prophecies that help them in a difficult situation, like dealing with a dark wizard who has a grudge against Harry even after the war is over.
Hermione would definitely be important. She could use her extensive knowledge to help Harry understand his creature inheritance. For example, she might research ancient texts to find out more about the origin of his inherited traits.
Hermione would definitely be an important character. She's smart and resourceful, so she'd be the one researching ways to restore Harry's memory. Ron would also be crucial as he's Harry's loyal friend. He'd be there to support Harry and keep him out of trouble. Dumbledore or other wise Hogwarts professors like McGonagall could also play significant roles. Dumbledore might have some secret knowledge or plan to help Harry, and McGonagall could provide guidance and protection at Hogwarts.
Sirius Black is an important new character. He is Harry's godfather. Also, Remus Lupin, the new Defense Against the Dark Arts teacher, is introduced.