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.
He may be confused at first. Fanfics can be very different from his real experiences. He might wonder why people write such things about him and his world.
Harry might initially be shocked and confused. He's used to Hogwarts being his safe haven, so when it turns against him, he'll be at a loss. He may try to prove his innocence by finding evidence or confronting those who are spreading lies.
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.
Well, Harry Potter is a rather humble character. He might initially feel a bit embarrassed to see himself as the center of so many fictional stories created by fans. If the fanfic portrays him as a hero in new adventures, he would probably be honored. However, if it misrepresents his friends or the values he holds dear, he could be a bit indignant. He would likely approach it with an open mind but also with a sense of protectiveness towards his own story and those he loves.
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.
Harry could also be angry. Despite understanding Dumbledore's generally good intentions, being tied to a bed against his will is a violation of his freedom. He might struggle against the restraints and demand to be released immediately, especially if he feels that Dumbledore is overstepping his boundaries.