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.
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.
Harry might be initially shocked and angry. He has always considered England his home and fought for it. So, being kicked out would be a huge blow. He may try to fight back, using his magic to resist the unjust decision. However, if he realizes that resistance is futile, he could leave with a sense of bitterness and a determination to one day clear his name.
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.
He could be initially shocked. Harry, who has trusted Ginny deeply, would find it hard to believe at first. He might go through a period of denial, trying to find excuses for Ginny's actions.
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.