In addition to the main trio from Harry Potter, Neville Longbottom could have a significant part. Merlin might see the potential in Neville and help him develop his skills further. And of course, there could be some new characters created specifically for this fanfiction that are related to Merlin's past or his new mission in the Harry Potter world.
Merlin would be the central figure. Ron Weasley could play an important role too. He might be a bit skeptical of Merlin at first but then come to respect him. Dumbledore could also be a major character, having some sort of connection or prior knowledge about Merlin's return.
One possible plot could be that Merlin, having been dormant for centuries, is awakened in the world of Harry Potter. He could then use his ancient magic to help Harry and his friends in their fight against Voldemort. Maybe he teaches them spells long forgotten, giving them an edge in the battle.
Harry Potter himself could be one of the main characters. He might be the one to discover the connection between his world and Merlin's through the presence of the Reaper. His natural curiosity and courage would make him a key figure in exploring this new mystery.
It could be a new student at Hogwarts. A young witch or wizard who discovers they have the Eyes of Merlin and has to deal with the sudden power and the attention it brings from both good and bad forces.
It's hard to say without reading it, but it could be the main characters from the Harry Potter series like Harry, Hermione, and Ron. They might be the ones exploring Merlin's legacy.
Often, there are other characters from the original Harry Potter series as well. For example, some fanfictions might include Hermione as a friend or mentor to the descendant. She can use her intelligence to help the main character understand their powers better. And of course, there could be some original characters created specifically for the fanfiction, like a mysterious wizard who also has a connection to Merlin's past.
There could also be a new villain who is specifically after the Merlin's descendant. This villain would be a major character, constantly creating obstacles and threats for the protagonist. They might have their own motives related to power, revenge or some ancient prophecy.
Another possible candidate in fanfictions could be Neville Longbottom. He has a deep connection to the magical world and shows great courage and growth throughout the series. His latent magical abilities, which are finally fully realized, could be interpreted as him being an heir of Merlin in a way. He has the potential to be a great wizard, just like Merlin was.
Definitely Harry Potter, who would be curious about this new development in the wizarding world. Neville Longbottom could also play a role, maybe he has some knowledge about the old dynasties that could help or hinder the king's plans. The Weasley family as a whole could be involved too. Ginny Weasley might have her own take on the situation. And there could be some ancient wizards who were thought to be long dead but are awakened with the king's return. These characters could add a lot of depth and mystery to the fanfiction.
Neville Longbottom can be a popular character in such fanfiction. In the original series, he grew into a strong and brave wizard, and in the fanfiction of Harry Potter's return, he could be a key figure in the wizarding world. Luna Lovegood might also be a popular character as her unique perspective and magical abilities can add interesting elements to the story.
Perhaps a powerful and corrupted spirit that guards Merlin's Castle and sees Harry and his friends as intruders. This spirit might have its own evil agenda and try to stop Harry from achieving whatever goal he has in the castle, like obtaining a powerful artifact or knowledge. It could use all sorts of dark magic and illusions to deceive and harm them.