The Cullens from the Twilight series are obvious choices. They could interact with Harry in various ways. For example, Carlisle's medical knowledge and his calm, father - like presence could provide stability in a dangerous situation that Harry and Edward find themselves in.
Sirius Black could also be an interesting character to include. His loyalty to Harry and his experience as an Animagus could add an extra layer of intrigue. He could perhaps teach Edward some tricks about being a non - traditional 'creature' or help Harry and Edward in a fight against a common foe with his combat skills.
A new character, a kind - hearted artist in the town, could be a great addition. This character could be the first to truly understand Edward's artistry with his scissorhands. They could collaborate on a project that changes the way the town views Edward, bringing more acceptance and new opportunities for him.
Alice could be a great addition. She could use her ability to foresee events to help Edward and Bella avoid getting caught by the police. Another good character would be Carlisle. He could be the voice of reason, trying to convince Edward to turn away from his criminal ways. Jasper might also be included, perhaps using his power to influence emotions to help in certain situations.
Definitely Sirius Black. Since he was one of Lupin's best friends and also had a strong connection with Harry. His presence can add a lot of drama and backstory to the fanfiction.
Well, Hermione would be a great addition. She could use her intelligence to help Harry and Moka understand the different aspects of their two worlds. For example, she could research about yokai and find ways to use magic to help Moka control her vampire side better.
Jacob would be a great character to include. He has a strong connection to the family, especially Renesmee. His werewolf nature could add an interesting dynamic to a crossover story. Also, Carlisle. His wisdom and experience as a vampire could be useful in different situations in a crossover. For example, if the crossover is with a supernatural world that has different rules about vampires.
Obviously, Harry and Gabrielle themselves are essential. But characters like Hermione can add a lot too. She could be the one to help Gabrielle with her studies when she visits Hogwarts. Ron can also be there for some comic relief and to show the difference between the Hogwarts and Beauxbatons students.
Definitely Dumbledore. His wisdom would be crucial in guiding Harry and Maxima in any situation. Hermione would also be great as she could research and figure out how Maxima's powers work in relation to magic. And of course, Hagrid, who could introduce Maxima to the more unique and dangerous magical creatures in the wizarding world.
Neville Longbottom is a wonderful character to add. He has grown so much throughout the series and can be a great support to the trio. Dumbledore, even if in a ghost - like or memory form, can offer his great wisdom. Also, Draco Malfoy can be included in an interesting way. Maybe in fanfiction, he has a change of heart and teams up with the trio against a greater evil.
Definitely Hermione. She's so intelligent and can offer a different perspective. She could help Teddy with his studies and give him advice on how to deal with the academic pressure at Hogwarts. Also, Ron. His sense of humor can lighten the mood. He could be like an older brother to Teddy, joking around with him and teaching him some fun wizarding tricks.
Hermione is always a great character to include. As a kid, she could be shown solving even more complex problems than in the books. Her intelligence and determination can drive the plot forward in fanfiction.