Many of the Hellsing characters would be initially skeptical of Harry. They are used to their own way of dealing with the supernatural, which mainly involves guns and brute force against vampires. Harry's magic, which is so different from what they know, would seem strange and perhaps even a bit untrustworthy. However, as they see Harry in action against common enemies, some might start to respect his abilities and come to accept him as part of the team. For example, some of the regular soldiers in Hellsing might be in awe of Harry's ability to conjure things out of thin air or use spells to defeat enemies that would otherwise be very difficult to handle with their usual weapons.
The Avengers' reactions would be diverse. Natasha Romanoff would likely assess Harry's skills and how he could fit into the team. Bruce Banner might be intrigued by the scientific aspect of magic. Clint Barton would be observing to see how Harry operates in combat. Overall, they'd be a bit skeptical at first but as they see what Harry can do, they'd welcome him. For example, when Harry first uses magic to save one of them from a dangerous situation, their doubts would start to fade.
Some of the SG1 characters might be initially skeptical. Daniel, being an archaeologist, would be very curious about Harry's magic. Jack might be a bit more cautious, but once he sees what Harry can do, he'd probably be more accepting. Teal'c would likely respect Harry's skills. Sam would be interested in how magic works and might even try to find a scientific explanation for it.
Hermione, on the other hand, would likely have a more critical view. She'd analyze the fanfictions for their accuracy in terms of magic lore and Hogwarts traditions. If there were any inaccuracies, she'd be quite perturbed. However, she might also appreciate the creativity that goes into some of the well - written ones that add new dimensions to their magical world.
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.