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.
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.
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.
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.
Another option could be Luna Lovegood. She has a very unique perspective on magic already. If she were Merlin's apprentice, she could bring her eccentric ideas and combine them with Merlin's ancient magic. Her friendship with Harry Potter and the others could also play a big role in how she uses her newfound status as Merlin's apprentice to help the wizarding world.
One interesting plot could be the discovery of Merlin's descendant having unique magical abilities that are different from the normal wizarding world powers. For example, they might be able to communicate with ancient magical creatures in a way no one else can.
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 might be a new transfer student from a far - off magical school. This student brings with them the knowledge or potential to be the new Merlin figure. They could have a different approach to magic compared to what's taught at Hogwarts, which makes them stand out as a possible Merlin - like character.
Obviously, the main characters would be Merlin, Harry Potter, and probably the founders would be mentioned or play some role in the background. After all, Merlin is from their era.
It could be a previously unknown student at Hogwarts. Maybe a shy Ravenclaw who has always been good at spells that require great intelligence and wisdom, which are traits often associated with Merlin.