Ron could also be involved. He might initially be a bit skeptical or jealous of the sister, but as they go through adventures together, they become friends. Maybe they get into some comical situations, like when they are trying to sneak into a restricted area of Hogwarts to find something related to the sister's forgotten past.
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.
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.
Obviously, Hagrid could be one. He would be so amazed and probably take baby Harry on little adventures around the Hogwarts grounds, listening intently to what Harry has to say.
A potential character could be a reclusive old wizard who lives on the outskirts of the wizarding world. This wizard might have his own reasons for living in seclusion, but when he sees Harry's loneliness, he takes him under his wing. Another character could be a young, orphaned witch who is also trying to find her place in the post - war wizarding world and she crosses paths with Harry.
There are many potential characters. The teachers at Forks High School for sure. Harry could have interesting conversations with them. Also, if there are any other supernatural beings in the Forks universe in this fanfiction, like werewolves, he could interact with them. And of course, the students who are into different clubs and activities at the school. He might bond with the science - loving students or those on the school newspaper team, sharing his own experiences from Hogwarts in a veiled way.
Obviously, Harry Potter himself would be the central character. Hermione could be part of the harem, as she has always been close to Harry. Luna Lovegood might also be there, with her unique and dreamy personality. Additionally, some new original characters could be introduced, like a powerful witch from a Vegas - based wizarding family.
There could also be new characters introduced. Maybe other magizoologists from different parts of the wizarding world. They could have different approaches and beliefs about magical creatures. Some might be rivals, trying to outdo Harry in the discovery and study of new species, while others could be mentors, guiding him in the complex world of magizoology.
Obviously, Hermione Granger would be a great addition. Her intelligence and loyalty would make her an important part of the harem. Ginny Weasley is also a likely candidate. She's brave and has a strong connection to Harry already. Luna Lovegood with her unique view of the world could add a lot of charm and mystery to the group. And Fleur Delacour, being part - Veela, could bring in an element of allure and different magical heritage.
Obviously, James and Lily Potter are key characters. Then there's Dumbledore. He was always involved in important matters, so he might be there to offer his wisdom or protection. Sirius Black could also be a character, as he was Harry's godfather. He might be hovering around nervously, eager to meet his godson. And Hagrid, who later played an important role in Harry's life, could perhaps be there to bring some news from the wizarding world to the Potters.
Definitely Sam and Dean Winchester. They would be like his surrogate brothers or mentors. Then there's Castiel, the angel. He could have some interesting interactions with Harry, perhaps being intrigued by his magic. Bobby Singer could also be there, guiding Harry in the ways of hunting.