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.
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.
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.
Hermione would probably be both curious and worried. She'd want to research all about Harry's half - demon nature to find out what it means and how to help him. Ron, on the other hand, might be a bit more hesitant at first. He might be a bit scared but would eventually come around because of his loyalty to Harry.
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.
Dumbledore would be very cautious. He would see Harry's reincarnation as a great risk but also an opportunity. He might try to guide Harry, but also keep a close eye on him to prevent any disasters. The Malfoys would likely see this as a chance to bring down Harry and his friends. They would spread rumors and try to turn the wizarding world against him. Harry's other friends from Gryffindor would be a mix of shocked, scared, and some would be brave enough to stand by him, believing that Harry is still the same person deep down.
Hermione would probably be very curious and immediately start researching ways to reverse the transformation or at least understand it better. She'd be quite protective of Harry too.
Well, Harry could react with a great deal of suspicion. Since he and Snape have had a rocky relationship, he might think Snape is up to no good. He could try to gather his friends, like Ron and Hermione, to help him figure out what Snape's real motives are. Harry might also try to use his own wits and magic skills to outsmart Snape if he feels that Snape is being unfair or dangerous in his forcing of actions. He could also be very conflicted, on one hand wanting to trust Snape as Dumbledore did, but on the other hand being very wary due to their past interactions.