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.
One possible plot could be Harry struggling to fit in at Harvard. He might be the only wizard there, so he has to hide his magical abilities while also trying to keep up with the intense academic curriculum. For example, he could use his quick thinking skills from his Hogwarts days to solve complex problems in his Harvard classes.
Perhaps there's a plot where Harry discovers a hidden magical mystery within the walls of Harvard. It could be related to an ancient artifact that has some connection to the wizarding world. He then has to use his magical skills, but also his newfound knowledge from Harvard to solve it.
Well, Harry is obviously one of the main characters. He's the one who is going to Harvard. There might also be some original characters created for the fanfiction. These could be Harry's classmates at Harvard, like a brilliant but eccentric roommate who helps Harry understand the ropes of the university.
Well, Harry could be really confused at first. Ouran is a very different world from Hogwarts. He might be shocked by the opulence of Ouran Academy and the strange club system there. Maybe he'd try to find a way to fit in while still being true to his wizard self.
He might meet international aurors. These are highly skilled wizards from different countries who are experts in dark wizard catching. They could teach Harry some new combat spells.
He could meet Fleur Delacour's family. Since Fleur is from France, her family might be an important part of the French wizarding community that Harry interacts with.
Hermione would be a likely companion. She loves learning and exploring, so she'd be eager to go on a traveling adventure with Harry. Ron might also tag along, especially if there's a chance of food and adventure. Neville could be another option. He has grown a lot in courage and his knowledge of plants could come in handy in different magical environments. Luna Lovegood would be a very interesting companion too. Her unique perspective on magic and the world would add a lot of fun and mystery to the journey.
Harry could meet some local magical creatures that are unique to Africa. For example, there could be a magical creature similar to a hippogriff but with different characteristics. And he may also meet African witches who are experts in herbal magic, which they use to heal and protect their communities. They could teach Harry some of their techniques and knowledge.