One main character could be a mysterious old wizard who has been living in the forest for centuries. He has a vast knowledge of the forest's magic and he takes Harry under his wing. Then there are the werewolves. If it's set during a time when werewolves are still feared, one of them could be in the forest. Harry could try to help this werewolf control their transformation, and they could become allies. Also, Neville could come to the forest to join Harry in some sort of magical research related to the forest's plants.
Hermione could be one. She might come to the forest to check on Harry and end up staying to study the unique flora and fauna which have magical properties. Another could be Hagrid. He loves all kinds of creatures and the forest is full of them, so he could visit Harry often and bring him supplies.
There could be forest animals like wise old owls or friendly squirrels that interact with him.
Maybe Harry discovers a hidden magical community in the forest. He could learn new spells from the forest creatures and use them to protect Hogwarts later.
Obviously, Harry Potter would be one of the main characters. There might also be his friends like Hermione and Ron, who could be involved in his adventures in the forest. And perhaps some new characters specific to the forest, such as a wise old tree spirit or a powerful forest guardian.
Harry Potter is the main character for sure. In addition, Dumbledore or other Hogwarts professors could make appearances. They might come to check on Harry or give him advice regarding his new life in Potter Manor. There could also be some family members from the Potter family tree, who were thought to be long - gone but now reappear in the form of memories or spirits in the manor.
Hermione could be one. She might follow Harry to the island to help him out. Another could be Ron, his loyal friend. Or it could be some new characters created specifically for this fanfiction, like a wise old wizard who has been living on the island for centuries.
Hermione could be one. She might follow Harry to Australia to continue their adventures together. She could be interested in studying the different magical plants in Australia and how they can be used in potions.
There could be an old French wizard named Alphonse. He might be a mentor to Harry, passing on his knowledge of ancient French magic. Also, a young French witch named Celeste could be a main character. She and Harry could develop a friendship or more, and she could be part of Harry's new life in France, like helping him understand the French magical education system which is different from Hogwarts.
Hermione Granger is often a main character. She might be exploring the forest to study its unique magical flora and fauna. Harry Potter himself could also be a central figure, perhaps using the forest's magic to enhance his own powers or to find a way to defeat a new dark force.
He might have adventures like befriending forest animals. Maybe he discovers a hidden magical spring in the forest that has special powers.
Often, Harry Potter himself can be a character in these fanfictions, as he has had many adventures in the Forbidden Forest. Hagrid might also be there as he has a deep connection with the forest and its creatures.