Maybe some of the Hogwarts professors could be involved. For example, Professor McGonagall could be concerned about Harry's new situation and offer her guidance. There could also be some descendants of previous island owners or caretakers who have their own interests in the island. They could either be friends or foes to Harry, adding more complexity to the story. Additionally, some magical creatures native to the island could be almost like characters themselves, interacting with Harry in unique ways.
Obviously, Harry himself is the central character. Then there could be his friends like Hermione and Ron. Hermione might help Harry manage his fortune in a smart way, like investing in magical books or rare potion ingredients. Ron could be the one who gives Harry more down - to - earth advice on how not to let the fortune change him.
Hermione would probably be involved. She could help Harry research about the island, like its history and any magical properties it might have.
One possible plot could be that Harry discovers the island is a magical place filled with ancient spells and potions. He might use it as a sanctuary away from the wizarding world's chaos, perhaps to study magic in peace and develop new spells.
In this fanfiction, the island could be a place where Harry's family had some sort of connection in the past. There could be a hidden family legacy there. Harry might find old family artifacts, and through exploring the island, he uncovers more about his family's history and how it ties into the wizarding world as a whole.
Neville could be in it too. He might be initially shocked to see Harry in such a state but then tries to help by using some of his herbology knowledge to make a potion to counteract the drunkenness. And Malfoy could show up, taking the opportunity to mock Harry until Harry, in his drunken craziness, does something that scares Malfoy off. Also, Hagrid would surely be around, being concerned about Harry and trying to get him to rest in his hut.
Hermione could be involved. She might follow Harry through time to try and stop him from becoming the Dark Lord or to fix the timeline if he already has. Dumbledore is another character. In the past, he could sense something strange about Harry's presence and try to guide him away from the dark path. Young Tom Riddle would definitely be involved. He could be either an ally or an adversary to Harry, depending on the plot. Maybe he sees Harry as a kindred spirit or as a threat to his own rise to power.
There could be a female leader on the island. She might be strong - willed and initially resistant to Harry's presence but later comes to respect him.
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.
Hermione is often a key character. She might stand by Harry's side, using her intelligence and loyalty to help him through the tough times. She could be the one to encourage him to keep going and not let the disownment break him.
Hermione is often in these fanfictions. She might help Harry understand the more complex magical aspects related to his broomstick. For example, she could research ancient spells that were once used to enhance broomstick speed and try to apply them to Harry's broom.