It could be that Dumbledore plays a more prominent role in this fanfiction. Since it's about 'the truth', perhaps Dumbledore has some secrets that are finally revealed, and he becomes a central figure in guiding Harry towards understanding. Additionally, some minor characters from the original series might be elevated to main characters if they hold key pieces of the truth.
Hermione Granger might also be a main character. Given her intelligence and thirst for knowledge, she could be researching the truth potion, its properties, and how to counter its effects if it's being misused.
Harry Potter himself is often a popular character in such fanfiction. His adventures and the secrets he could potentially reveal in truth or dare make him interesting. Hermione is also popular as she has a lot of knowledge and secrets. Ron, with his humorous nature, is another common choice.
Harry Potter himself is often a central character. The truth serum might be used on him, or he could be the one trying to find out the truth from others. Hermione Granger is also common, as her intelligence might be used to either create or counteract the effects of the truth serum.
Obviously, Harry, Hermione and Ron are the most likely. They are the main characters and are always together, so it's natural for them to be playing truth or dare. Neville could also be involved as he's their friend. And Ginny might join in too, especially as she gets older and becomes closer to the group.
Another possibility is a new character created specifically for this fanfiction. This character could be a scientist or a wizard from a different part of the world who has knowledge of both nanotechnology and magic, and they bring the concept of nanobots into the Harry Potter universe.
Obviously, Harry Potter and Marris are the main characters. Harry is the well - known boy wizard, and Marris is the character specific to this fanfiction. There may also be other characters from the original Harry Potter series like Hermione and Ron, who might play supporting roles in the fanfiction.
Obviously, the grandchildren of Harry Potter are main characters. They often have names that are related to the Potter family traditions. There could also be their friends at Hogwarts, who might be from different wizarding families or even Muggle - borns.
Most likely, Harry Potter would still be one of the main characters. Since it's in the Harry Potter universe, he's central to the story. He could be the one around whom the new bonds are formed or strengthened.
In addition to the ones mentioned, Dumbledore could be a main character in some fanfictions. He knew Flamel, so he could play an important role in stories about Flamel. Voldemort could also be a main character as he was after the Philosopher's Stone, and his actions could be intertwined with Flamel's story. There could also be original characters created by the fanfiction authors who interact with Flamel and the other known characters.
There could be new characters created specifically for this fanfiction that are central to the merging concept. For example, a character who has the ability to control the merged magic or someone who is the key to uniting different groups of wizards that were brought together in this fan - made story.