He could be shocked. Maybe there are some fanfictions that take a very dark turn or have unexpected pairings that he never thought of. He'd probably need some time to process such strange ideas. And he might also be curious to see how far the creativity of the fans can go. He might even get some new ideas about his own life from reading some of the more imaginative fanfictions.
Hermione, on the other hand, would likely have a more critical view. She'd analyze the fanfictions for their accuracy in terms of magic lore and Hogwarts traditions. If there were any inaccuracies, she'd be quite perturbed. However, she might also appreciate the creativity that goes into some of the well - written ones that add new dimensions to their magical world.
There's a chance he'd be inspired. The stories of the Naruto characters' determination and growth in the fanfiction could stir something in him. He might think about his own journey as a wizard and how he can further develop his skills and character, taking cues from the Naruto characters' tenacity.
The headmaster might be a bit surprised at first. He would likely respect Harry's opinion, though. He has always valued Harry's thoughts and feelings. He may start to think about the implications of fanfiction on the wizarding community and how it might affect the students and the overall perception of Hogwarts.
As time goes on, Harry could start to explore what it means to 'please' Sirius. He might recall their past relationship, the things Sirius likes and dislikes, and try to use that knowledge to his advantage. He may also feel a sense of loyalty towards Sirius that, despite the odd circumstances, makes him more accepting of the situation. For example, he might remember Sirius' love for adventure and try to engage in some magical adventures with him in a way that 'pleases' him.
At the beginning, Harry would be frightened. He's in an unknown place with Loki, who is known for his trickery. But Harry is no pushover. He would quickly gather his courage. He would remember all the times he's faced danger at Hogwarts. He might try to communicate with Loki to understand his motives. If Loki reveals that he wants to use Harry's magic for something, Harry would try to find a way to outsmart Loki. He could pretend to cooperate while actually looking for a weakness in Loki's plan. Harry would also be thinking about his friends at Hogwarts and how he needs to get back to them, which would give him extra motivation to fight back against Loki.
The characters in Harry Potter might be a bit shocked at first. Hermione would probably be very curious about Supergirl's powers and how they work. Ron might be a bit jealous at first but would eventually come around. Dumbledore, if he were still alive, would likely see it as an opportunity for the wizarding world to expand its horizons. In the Supergirl world, Kara's friends like Alex would be protective at first but would see Harry's good heart. J'onn J'onzz would be intrigued by the magic and how it could be used in their fight against evil.
Well, the Weasley family would have a mixed reaction. Molly Weasley would be concerned about how this new relationship would affect Harry and whether Medea was a suitable partner. Ginny might be a bit jealous at first but then come to accept it. In the Hogwarts faculty, Professor McGonagall would be very strict in her assessment. She would want to ensure that Medea adhered to the rules and ethics of the wizarding world. The other houses, like Hufflepuff, might be more accepting overall, seeing it as a way to bring more diversity to the magical community. Slytherins, on the other hand, would be divided. Some would see it as an opportunity to gain an advantage through this new connection, while others would oppose it out of pure prejudice.