There could be forest animals like wise old owls or friendly squirrels that interact with him.
One character could be Dobby. Even though Harry is living alone, Dobby might still find a way to visit him and offer his help. Another could be the Hogwarts ghosts. They can pass through walls and might visit Harry in his solitary abode.
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.
He might have adventures like befriending forest animals. Maybe he discovers a hidden magical spring in the forest that has special powers.
One of the main characters would likely be Death itself, who could take on different forms and interact with Harry throughout his life. Dumbledore would also be an important character as he tries to understand and guide Harry, despite his unusual upbringing. And then there are Harry's friends like Hermione and Ron, who would have to adjust to Harry's different perspective on life and magic due to his being raised by Death.
Ron and Hermione are very likely to interact with Harry. They are his closest friends in the original series, so in the fanfiction, they would still be important characters in his life. They might be in awe of his new all - powerful abilities or they could also be worried about how the power is changing 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, Buffy would be a major character he'd interact with. They could start off as wary of each other but then grow to respect one another's fighting skills. Giles too, as he has a wealth of knowledge about the supernatural, and he and Harry could have some interesting discussions about different types of magic.
Hermione is usually one of the main characters that interact with Harry. She might be in awe of his new powers at first but then help him use them wisely. Ron could also be there, perhaps being a bit more skeptical at the start but eventually coming around. Dumbledore might also play a role, guiding Harry even in his god - like state, sharing some of his own deep knowledge of magic.
Hermione would surely be one of the main characters interacting with Harry. She might try to bring him back from the dark side, constantly arguing with him about his choices and trying to remind him of their old friendship and the good they used to fight for.
The wizard Gandalf would be an obvious character for Harry to interact with. They are both wizards, though from different worlds. They could share their knowledge of magic, and Gandalf could guide Harry in the ways of Middle Earth while Harry could introduce some new magical concepts to Gandalf.