The girl might be a descendant of an old wizarding family that has fallen on hard times. Harry adopts her to revive the family's honor. She could have some special knowledge or heirloom that attracts Harry's attention and makes him decide to take her in.
Perhaps the girl is a huge fan of Harry Potter in the fanfiction world. When Harry discovers her, she's being bullied for her love of him and his story. So, he adopts her to protect her. He then shows her the real Hogwarts, not just the one she's read about. Along the way, they might face challenges from those who don't like the idea of Harry having an adopted daughter, but they overcome them together.
She could be a young girl who has lost her parents in a magical accident. Maybe she has some special powers that Harry notices and that's why he decides to adopt her.
Perhaps Bruce Banner adopts Harry Potter after the events at Hogwarts. He takes Harry to the world of science, showing him things like advanced laboratories and scientific research. Harry, in turn, shows Bruce the wonders of magic. Their bond grows as they learn from each other, and together they face new threats that combine elements of both the magical and scientific worlds.
It could be a young wizard who lost their parents during the final battle against Voldemort. Harry feels responsible for all the orphans created by the war and decides to take this particular child under his wing.
One possible plot is that the girl is from a non - magical family but has a deep fascination with Harry's story. Harry discovers her and decides to adopt her as she has a natural inclination towards magic. He trains her privately at first, and as she gets better, he reveals her to the wizarding community. Along the way, they face some challenges from those who are against a Muggle - born being trained by Harry, but they overcome them together.
Another theme is self - discovery for Harry. With the influence of the MC, Harry gets to explore different aspects of himself. For example, he might discover new magical abilities or gain more confidence in his identity as a wizard. In some fanfictions, the MC helps Harry to look beyond what he thought was possible for himself.
Well, one possibility is that Harry realizes Fenrir's tragic backstory and decides to help him change. Maybe he uses magic to suppress Fenrir's werewolf side and tries to integrate him into the wizarding society. Harry could teach Fenrir about friendship and loyalty, and together they fight against the dark forces that once exploited Fenrir.
The combination of two very different universes. Ichigo from Bleach is a very different character from those in Harry Potter. His unique powers and personality, when put in the context of the wizarding world, create a lot of interesting scenarios.
Well, Elrond could use his ancient wisdom to train Harry in magic that's different from Hogwarts. Maybe he'd teach Harry about elven magic, which could give Harry an edge in fighting Voldemort. Elrond's knowledge of ancient spells and potions could be a great asset to Harry.