The main characters are clearly young Joseph and Dio. Joseph is a character with a strong will and unique abilities, especially his Hamon. Dio, on the other hand, is a powerful vampire with his own set of formidable powers.
Perhaps a plot point where the power dynamic between them shifts unexpectedly. Maybe Joseph discovers a weakness of Dio that no one else knew, leading to a very tense and exciting confrontation. In fanfiction, this could be something completely new and creative that the author invents.
The determination of whether the novel 'Dio' is canon can be complex. It might involve factors like consistency with established lore, authorial intent, and the response from the fan community. Without specific details, it's hard to give a definite answer.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.