Hermione would be horrified. She's always by the book, and summoning Cthulhu is the most dangerous and unthinkable thing. She'd immediately start researching ways to send it back. Ron would be scared out of his wits but also a bit in awe of the power. Harry would be conflicted, realizing he's brought something extremely dangerous into their world.
The teachers at Hogwarts would have a range of reactions. Dumbledore might be more intrigued than afraid at first, trying to understand the nature of Cthulhu. Snape would be disgusted that Harry had done something so reckless. Hagrid would be both scared and sympathetic, as he has a soft spot for all creatures, but he'd know this one is different. The students would be mostly terrified, some might even try to run away from Hogwarts, fearing for their lives.
The plot might involve Harry using his magic in an unorthodox way to summon Cthulhu. Maybe he stumbles upon an ancient spell in the restricted section of the library. Once summoned, Cthulhu could bring chaos to the wizarding world. The dark and eldritch powers of Cthulhu would be in stark contrast to the more traditional magic in Harry Potter. This could lead to a struggle between the forces of Hogwarts and the indescribable horror of Cthulhu.
When Harry summons Echidna, the Hogwarts environment would experience a shockwave of magic. The plants in the greenhouse might wither or grow uncontrollably depending on the nature of Echidna's magic. The House - elves in the kitchen would be on high alert, hiding their precious food supplies. The centaurs in the forest would be conflicted; some might see it as a sign of great change and danger, while others might be intrigued by the appearance of such an ancient being. The Hogwarts faculty would quickly gather to form a protective perimeter around the students, not knowing exactly what Echidna might do.
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.
Well, Harry would be much more aloof and menacing. He might not interact with Hermione and Ron in a friendly way at all. Instead, he could view them as potential sacrifices or just insignificant beings. Hermione, being the intelligent one, might be terrified of Harry's newfound powers and strange upbringing. Ron would probably be outright hostile towards Harry, seeing him as a threat to the normal wizarding way of life.
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.