Who are the main characters in 'Harry is a elf fanfiction' other than Harry?Ron could also be a main character. He might initially be a bit confused or even jealous of Harry's new powers as an elf, but then come around to support him. His family, like the Weasleys, could also be involved, perhaps providing a different perspective on elves compared to the pure - blood wizarding families.
Who are the main characters in 'harry is a prodigy fanfiction' other than Harry?It's hard to say for sure without reading the fanfiction, but it's likely that Ron and Hermione are still important characters. They are Harry's closest friends in the original series, and their relationships with a prodigy - Harry would be interesting to explore. Maybe they have to deal with feeling overshadowed by Harry's new - found prodigious abilities.
Who could be the main characters in 'harry is mated to a vampire fanfiction' other than Harry?A vampire hunter could be a key character. They might initially see Harry as a target too because of his association with the vampire. This would create a lot of tension as Harry and his mate have to either convince the hunter that they are not a threat or fight against them. And don't forget about Dumbledore (assuming this is in the Harry Potter universe). His reaction to Harry's new situation would be very interesting to explore.