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 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.
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 Slytherin house would probably be outraged. Snape is one of their own, in a sense. They would demand justice for Snape and might even use this incident to fuel their long - standing rivalry with Gryffindor. There would be a lot of tension and conflict within the Hogwarts community as a result of this act.
Some characters in Westeros might be very suspicious of Harry. His strange clothes and the way he does magic would make them think he's some sort of dark sorcerer. Others, like the more open - minded ones, might be curious and want to learn from him. For example, Tyrion Lannister might see Harry as an interesting oddity and try to figure out how his magic works.
Perhaps the travel agent is a bit of a troublemaker. Instead of suggesting normal wizarding travel destinations, they might try to persuade Harry to visit some off - limit or even illegal magical places. They could tell him about a secret portal to a dimension where dark magic is still being practiced freely, which of course, Harry would have to resist.