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.
Dumbledore would be an interesting character in this fanfiction. His wisdom and powerful magic could be crucial in dealing with any major threats that might come up in the story. He could also offer guidance to Moka about her powers and how they can be used for good in a different context than she is used to.
Definitely Sasuke should be included. He could have some interesting interactions with Inner Moka. Maybe he's initially suspicious of her vampire nature but later comes to respect her skills. Sakura could also be a great addition. She could bond with Inner Moka over their shared strength and determination. Kakashi would be another good choice. His calm and experienced demeanor could contrast well with Inner Moka's more impulsive nature.
Kurumu would be a great character to include. She has a crush on Tsukune and her jealousy towards Moka could add some interesting drama to the side story. Also, Mizore, with her cold exterior but warm heart towards Tsukune, could bring in some unique interactions. Gin, being a werewolf, could also be included as he has an uneasy relationship with the vampires, which could create tension and conflict in the side story.
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.
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.
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.
Adding Draco Malfoy could create some interesting drama. He could use Harry and Ginny's jealous games to his advantage, maybe trying to drive a wedge between them. And of course, the Weasley family as a whole. Molly Weasley could notice Ginny's behavior and have a motherly talk with her, while the other Weasley siblings could have different reactions to the whole jealous situation between Harry and Ginny.