Some characters might be very curious. Clark, for instance, would probably be interested in Harry's magic since he has his own superpowers. He'd want to know where Harry's powers come from and how they work.
Well, it could be really interesting. Harry might use his magic to solve some of Smallville's strange problems that are usually dealt with in more 'normal' ways in the show. He could also introduce the people of Smallville to a whole new world of magic.
Some characters might be excited. They could see him as a new source of knowledge and magic.
The Weasley family would have different responses. Mrs. Weasley would be motherly and concerned about this strange new person. Fred and George would see her as a source of new ideas for their pranks and inventions. Ginny would look up to her as a strong female figure, much like she does with Hermione. And all of them together would gradually get used to having Powergirl around, and they would all grow from the experience of interacting with someone from a completely different world.
Batman would probably be very cautious at first. He'd want to study Harry and his magic to see if it could be a threat or an asset. The Joker might see Harry as a new plaything, a strange and magical being to toy with. Commissioner Gordon would likely be intrigued and hope that Harry could help clean up Gotham's streets.
Some characters might be quite amused. For example, Hermione would probably analyze the fanfiction from a logical perspective, critiquing the plotlines that involve ships. Ron, on the other hand, might be a bit bewildered at first but then find it rather entertaining. Harry himself could be either indifferent or intrigued, depending on how the ships are portrayed. If it's a ship that he has a personal connection to, like his parents, he might be more interested in seeing how the author has envisioned their relationship in the fanfiction world.
Dumbledore would likely be very accepting of Stitch. He has a great understanding of all kinds of creatures and beings. He would see Stitch's potential for good and might even try to find a place for him at Hogwarts. Snape, however, would be more skeptical. He would keep a close eye on Stitch, worried that he might disrupt the order of the school or cause harm to the students. The Weasley twins would probably find Stitch hilarious and would want to play pranks with him or use his unique abilities for their own mischievous plans.
Draco Malfoy would probably be outraged at first if he read fanfiction that showed him in a positive light towards Harry and his friends. He's so used to being on the opposite side, but then he might secretly be a bit flattered by some of the more complex characterizations of him.
In a fanfiction, the students at Hogwarts would be really intrigued by Hawkeye. They've grown up in a world of magic, and Hawkeye would represent a different kind of power. Neville might look up to Hawkeye as someone who uses their own skills to fight, without relying on magic. Ginny would probably be eager to team up with Hawkeye in any battles or adventures, seeing his potential as an ally in the fight against evil.
Hermione would probably be very curious about Darkseid at first. She'd try to research him in the Hogwarts library, looking for any clues about his powers and weaknesses. But once she realizes the danger he poses, she'd be quick to come up with strategies to fight him.
Hermione would be extremely curious. She would immediately start researching it, trying to figure out its mechanics and where it came from. She might even try to find a way to improve it or adapt it to the wizarding world's technology.