Obviously, Harry would be one of the main characters. There could also be local Harlem residents like a street - smart kid who shows Harry around. A jazz singer could be another character, perhaps someone with a mysterious past that Harry gets involved with.
Obviously, Harry Potter himself. And there could be characters like Hermione and Ron who might either go with him or be involved in some way from afar. There may also be original characters created for Alfe, such as an Alfe wizard who becomes Harry's guide.
Obviously, Harry Potter himself. Then there could be Hermione and Ron, who might visit him at Harvard or communicate with him via magic. There could also be new characters like his Harvard roommate, who could be a normal Muggle but start to suspect that Harry is not an ordinary guy. And maybe some professors at Harvard who take an interest in Harry's different way of thinking.
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.
The plot could involve Harry going to Harlem in search of something specific, perhaps a magical artifact if we're adding a bit of magic to the fanfic. Along the way, he meets interesting characters who help or hinder his search. He could also experience the different food and art in Harlem, which changes his perspective on life.
In this 'harry potter goes to the sixth world fanfiction', we would expect Harry Potter. Besides him, there might be new wizards or witches native to the sixth world. They could have their own unique powers and personalities. Maybe there are also some half - magical beings that Harry has never encountered before. These new characters would add more complexity and freshness to the story.
Obviously, Harry Potter himself. There would also likely be Viktor Krum, who is from Durmstrang. Krum could be either a friend or a rival to Harry. Igor Karkaroff, the headmaster of Durmstrang, would be another important character. And there could be new students created for the fanfic, like a Durmstrang student who becomes Harry's guide in the new school.
Sakura might be one of the characters. She has her own set of skills in the medical and combat fields. Meeting Harry could lead to exchanges of knowledge about different forms of healing and combat magic.
He could meet the main characters from Death City. For example, Maka Albarn and Soul Eater Evans. They could be initially cautious around Harry but then become friends as they realize his good intentions.
Obviously, Harry Potter himself. There could also be some Beautox - natives, like a wizard or witch who is an expert in Beautox magic. Maybe Hermione would somehow find a way to follow Harry and she'd be there too, trying to figure out the new magic system.
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.