Ron Weasley and Puck could be great together. Ron is the loyal friend in Harry Potter series, and Puck has his own charm in Glee. They might get into some comical situations. Maybe Ron tries to learn how to play the guitar from Puck, and Puck is constantly amazed by the magical world Ron comes from. Their friendship could be a source of humor and also a way to bridge the two different universes.
Definitely Harry Potter and Finn Hudson. Harry is the hero in the wizarding world, and Finn is a key figure in the Glee club. They could bond over their leadership roles. Harry could teach Finn some magic tricks, and Finn could show Harry how to be more confident while performing in front of a large audience.
Hermione Granger and Rachel Berry would have an interesting interaction. Hermione is the brilliant and studious one in Hogwarts, while Rachel is the diva in Glee. They could start off as rivals, both being very competitive. But then they realize they can learn from each other. Hermione could help Rachel with her academic knowledge, and Rachel could give Hermione some tips on stage presence.
Probably Harry Potter and Edward Cullen. Since Edward is sort of the main character among the Cullens in a way, and Harry is the central figure in his world. They both have unique abilities and a sense of responsibility, so it makes sense for them to be the first to cross paths and start interacting.
Hermione and Betty would be likely to interact. Hermione's intelligence and thirst for knowledge could pair well with Betty's investigative nature. They could work together to solve a mystery that spans both the wizarding world and Riverdale.
Neville could be another character. He might look up to Harry even more in his dragon form, seeing him as an even more powerful protector. Hagrid would be excited to have a dragon - Harry around. He could help Harry understand his new dragon nature. And some of the Slytherins, like Draco, might be jealous or scared of Harry's new form.
Most likely, it would be Harry Potter and Nico di Angelo. Harry is the main character in his world and is always the one to step forward when something strange is going on. Nico, being the son of Hades, has a certain air of mystery around him. So, when he appears in the wizarding world, Harry would be the one to approach him out of curiosity and a sense of duty to protect Hogwarts from any potential threat.
The main characters would surely include Harry Potter. Since it's about him meeting the founders, he's central to the story. As for the founders, they are key too. Godric Gryffindor might be the most enthusiastic about meeting Harry, seeing in him the true spirit of Gryffindor. Helga Hufflepuff could be the nurturing one, teaching Harry about the importance of hard work and loyalty. Rowena Ravenclaw might offer him wisdom, and Salazar Slytherin could present a more complex relationship, as his views on blood purity might clash with Harry's, but still be an important part of the story as Harry tries to understand and reconcile different magical ideologies.
Obviously Gibbs himself would be a main character interacting with Harry. He'd be like a father figure.
Snape would have a very different relationship with Harry in this scenario. Instead of being a constant source of disdain, he might see Harry as an opportunity to shape a Slytherin in a new way. Pansy Parkinson could also be involved. She might initially be against Harry being in Slytherin but later come to accept him, or they could be in a constant battle of wits.
Voldemort would be a likely character to interact with Harry. Since Bellatrix was one of Voldemort's most loyal followers, he might take an interest in Harry as her son. He could either see Harry as a potential powerful ally or a threat depending on how his powers develop. Another character could be Draco Malfoy. Given the connection between the Malfoy family and the Death Eaters, Draco might have a complex relationship with Harry, perhaps initially seeing him as an equal in the dark magic world but later having doubts as Harry shows signs of not fully embracing the dark side.
In a 'Harry Potter meets Apollo' fanfiction, their interaction would be quite complex. For starters, Dumbledore would surely approach Apollo with great respect, being aware of the power and significance of a god. Apollo might be interested in Dumbledore's wisdom and magic. The students, like Neville, could look up to Apollo as a new source of strength. Meanwhile, Apollo might see the potential in these young wizards and witches and decide to mentor them in some ways, sharing his knowledge of combat and strategy from his long existence in Greek mythology.
Hermione would probably try to understand Shendu from a logical and magical perspective. She could attempt to communicate with him to find out more about his powers and his weaknesses. Ron, on the other hand, would be more likely to jump into action if he saw Shendu as a threat. He might not be as diplomatic as Hermione and could end up getting into a scuffle with Shendu's minions. The three of them together would have to figure out how to deal with Shendu's different approach to magic compared to what they're used to in the wizarding world.