Voldemort would surely be involved. He might initially be suspicious of Harry's sudden change but then see it as an opportunity to use Harry's power for his own dark ends. He could try to mentor Harry in the ways of dark magic.
The Death Eaters would be interested in Harry. They would either see him as a threat or a potential ally. Some of the more ambitious Death Eaters might try to manipulate Harry for their own rise in the dark hierarchy, while others might be afraid of his power and want to eliminate him.
Obviously, Harry Potter himself would be the central character as the title implies. Hermione and Ron would most likely be involved as well, since they are Harry's closest friends. They would probably be the ones trying to figure out what's wrong with Harry and how to fix him. Dumbledore could also play a big role, as he often has the wisdom and power to deal with strange situations.
There could be some new characters created specifically for this fanfiction. For example, an old wizard who lives near the fishing area and gives Harry some tips on magical fishing. Or perhaps a mermaid who tries to play tricks on Harry while he's fishing. Hagrid might also be involved, as he loves all kinds of magical creatures and fishing could be a new adventure for him too. He could bring along some of his strange pets to the fishing trip.
Obviously, Harry himself is the central character. Hermione would probably be involved too. She might try to reason with Harry at first but then end up supporting him. Ron would also be there, likely being a bit more hesitant but still standing by his friends.
Dumbledore could make an appearance. He might be the one who gives Harry the idea to become a pilot in the first place or provides some ancient magical knowledge about flying that helps Harry in his new career. Also, the Weasley twins could be involved. They could create some magical gadgets for the plane, like a device that makes the plane do cool stunts.
Obviously, Harry Potter himself would be the central character. Then, Voldemort would be a major figure as he is the main representative of the Dark Side. Bellatrix Lestrange could also play an important role as she is one of Voldemort's most loyal followers. She might be the one to initially tempt Harry or guide him into the ways of the Dark Side.
Hermione and Ron would definitely be main characters. They would be the ones most affected by Harry's decision to join the dark side. They would likely try to figure out why he made such a choice and try to stop him. Dumbledore, even if he's dead in the original story, could still have an influence through memories or prophecies. Voldemort would also be a central figure as he is the main representative of the dark side that Harry is joining.
Hermione would likely be there. She's Harry's best friend and would not let him be in juvie alone. She'd probably be trying to find a way to get him out legally, researching all the laws and loopholes. Ron might also show up, being another one of Harry's closest friends. He could be the more impulsive one, thinking about breaking Harry out rather than going through the proper channels. Dumbledore might also be involved in some way. He has always been a guiding figure for Harry, so he could be working behind the scenes to ensure Harry's safety and find a way to clear his name.
Ron might also be there. He could be the one who offers Harry a place to stay at his own home if Harry is having trouble finding a place. Or, if they've had an argument, Ron could be the reason Harry is moving out in the first place.
Hermione would probably be involved. She's always by Harry's side, and she might be researching ways to help Harry tame the basilisk. Maybe she finds some useful spells or information in the library that gives Harry an edge.
Obviously, Harry Potter himself is the central character as the one being abused and neglected. The Dursleys are also key characters as they are the ones who abuse and neglect him in the original story. Hermione and Ron might also be involved as they could be the ones to discover Harry's situation and try to help him. Dumbledore could also be a part of it, perhaps as someone who failed to notice or prevent Harry's abuse initially.