In the fanfiction, Merlin could interact with the ghosts of Hogwarts. Since he has a long history, he might have known some of the ghosts from his own time or have some connection to them. He could also interact with Hagrid. Hagrid would be in awe of Merlin and would love to show him all the magical creatures at Hogwarts. And Merlin might offer some unique insights on how to better care for these creatures based on his own experiences with magical beasts in his era.
He would probably interact with Dumbledore. They are both very wise, and Dumbledore would be eager to learn from Merlin's ancient knowledge.
He might initially be met with suspicion. The Hogwarts faculty are protective of their school and students. However, Percy could win them over with his courage. For example, if there was a dangerous situation, he would jump in to help, just like he did in his demigod adventures. Professors like McGonagall might be impressed by his loyalty and sense of justice.
In this fanfiction, Dudley would surely interact with the Hogwarts professors. Professors like McGonagall would be strict with him, making sure he follows the rules. He might also cross paths with Draco Malfoy. Given Malfoy's attitude towards Muggle - borns, there could be some conflict there. And let's not forget the other students in his house. They would play a big role in his Hogwarts experience, either befriending or ostracizing him.
Another possible friend for Danny could be Hermione Granger. Hermione is always interested in learning new things. Danny's powers would be a fascinating subject for her to study. She might try to find a way to explain his powers through magic or Muggle science. And Danny could benefit from Hermione's vast knowledge of magic and Hogwarts.
She'd probably be a bit of a misfit in Hufflepuff. The values of hard work and loyalty in Hufflepuff might not seem as exciting to her as the action - packed world she's used to. In Gryffindor, she'd fit in better with their courage, but might also clash with some of the more reckless Gryffindors. In Ravenclaw, she could learn a lot from their thirst for knowledge. And in Slytherin, she might understand their ambition, but also be at odds with some of their more underhanded tactics.
Well, he might show off some really old - fashioned but extremely powerful magic that would shock everyone at Hogwarts. Maybe he'd befriend Harry Potter and teach him some ancient spells that could defeat Voldemort in a whole new way.
He would probably interact with Dumbledore a lot. They are both very powerful wizards and could share a great deal of magical knowledge.
Sakura could have interactions with the Weasley family. Ginny Weasley might become her friend and they could have girl - talk about magic and boys. Fred and George could also be part of her story. They might play some pranks on her at first but then become good friends. And don't forget Neville Longbottom. Sakura could help Neville gain more confidence in his magic abilities as they study together in the Hogwarts library.
He would probably interact with the Hogwarts professors. For instance, he could have discussions with Dumbledore about different forms of power - his ninja chakra and Hogwarts magic. He might also work closely with McGonagall, who could be interested in Naruto's shape - shifting abilities compared to her Animagus form.
Sakura might become friends with Hermione. They both are intelligent and interested in learning, so they could bond over books and studying magic. Sasuke, well, he might be a bit more aloof, but he could end up having an odd friendship with Harry. They both have a sense of responsibility and a past filled with challenges, so they might understand each other on some level.
Harry Potter himself would be intrigued. Given his role in fighting dark magic, he'd be eager to learn from Merlin, who has centuries of magical experience. Merlin could teach Harry new ways to defend against dark magic that are different from what he's learned from Dumbledore and other professors.