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.
Another possibility is that it could be a character who was previously thought to be a minor antagonist but then gains the Eyes of Merlin. This character then has to decide whether to use the power for redemption or to continue down their dark path. Their internal struggle between good and evil while dealing with the power of the Eyes of Merlin could be a really engaging story.
Harry Potter might still be the main character even with Merlin's return. Merlin could be more of a guiding or helping figure, and Harry would be the one who ultimately has to face the main challenges with Merlin's assistance. So Harry would remain at the core of the story.
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.
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.
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.
Perhaps an old follower of Voldemort who was in hiding. This person finds out about the Eyes of Death and believes that with their power, he can bring back Voldemort's legacy even stronger. He becomes the main force against Harry and his friends throughout the fanfiction.
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.
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.
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.
Maybe it could be considered so because of the magic theme. Merlin is well - known for his magic, and in 'Harry Potter', magic is everywhere. The idea of a powerful wizard guiding the fate of others, like Dumbledore in 'Harry Potter' could be seen as similar to Merlin's role in his stories. But this is a very loose connection.
How about Neville Longbottom? He could be the main character in a fanfiction set in the Chamber. Neville, who has always been underestimated, could show his true courage and strength. He might discover a hidden ability that helps him face the challenges in the Chamber, perhaps something related to his connection with plants, which could play a crucial role in defeating the Basilisk or whatever new threat is in the Chamber.