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.
Merlin could also be a main character, especially if the fanfiction involves time - travel or some sort of connection between the past and present. He could bring his ancient magic and wisdom to the Harry Potter world, and his relationship with the Reaper might be something that has been long forgotten but is now resurfacing.
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.
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.
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.
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.
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.
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.
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 might be a new character introduced in the fanfiction who is using the appearance of the Grim Reaper to cause chaos. This character could be a dark wizard who has found a way to mimic the Grim Reaper's powers and uses it to spread fear among the wizarding community. Harry would then have to figure out who this impostor is and stop their evil deeds.
Well, clearly Merlin and Harry Potter are the main characters. Merlin, with his powerful magic and wisdom from a different era, and Harry, the boy - who - lived from the modern wizarding world.
Well, Harry Potter would be the main character of course, given that he is the 'Son of Merlin' in this fanfic. His friends Ron and Hermione are likely to be important characters too, as they are always by his side in the original series and would probably play significant roles here as well.