In the context of the Harry Potter universe, the founders are Godric Gryffindor, Salazar Slytherin, Rowena Ravenclaw and Helga Hufflepuff. In this fanfiction, it's likely these are the founders who are training Harry.
The founders in the 'harry is trained by the founders' fanfiction are the original creators of Hogwarts. Godric Gryffindor, a brave and noble wizard. Salazar Slytherin, with his complex views on magic and wizardkind. Rowena Ravenclaw, the brilliant mind behind Ravenclaw house. And Helga Hufflepuff, who welcomed all kinds of students into her house. Since they founded Hogwarts, they must have had vast knowledge and power, and in this fanfiction, they are using it to train Harry.
I'm not sure specifically as there could be many different plots in such a fanfiction. It might involve the founders of Hogwarts (like Godric Gryffindor, Salazar Slytherin etc.) training Harry in magic skills, perhaps in areas that the regular Hogwarts curriculum doesn't cover fully, like advanced dueling techniques or ancient magic knowledge.
It could be Harry Potter himself. Since he has so many special qualities and a deep connection to Hogwarts, he might be revealed as an heir. Maybe he has some latent powers related to one of the founders that haven't been discovered yet.
Neville Longbottom is another possible heir. He has a strong connection to Gryffindor through his family's long - standing association with the house. His growth from a shy and clumsy boy to a brave and powerful wizard could be part of a story where he is the heir of the founders. His ability to stand up to the Death Eaters in the final battle shows his hidden strength, which could be related to the founders' magic.
Harry Potter himself could be a likely candidate. He has shown many qualities that the founders would have admired, like his courage and loyalty. Also, he has a connection to the Sorting Hat which has knowledge of the founders' intentions. Another possibility could be Hermione. Her intelligence and thirst for knowledge are traits that could be associated with the founders. She could use her knowledge to uncover secrets related to the founders and their heirship.
Without more context, it's impossible to say who exactly these founders are. They could be founders of a small online Harry Potter fan community, or perhaps founders of a local Harry Potter reading group that also dabbles in fanfiction.
In a large number of fanfictions, Harry is indeed made to be descended from the founders. This concept allows for a great deal of creativity. It could mean that he has an innate understanding of the magic that the founders were known for. Maybe he has a natural affinity for the secrets of Hogwarts if he's related to Helga Hufflepuff, or perhaps he has a more political influence within the wizarding world if his ancestry is linked to Rowena Ravenclaw. Fanfiction writers use this idea to expand on the existing Harry Potter universe in unique and exciting ways.
Well, obviously Harry would be the main character. Since he's the heir of the founders in this fanfiction.
Obviously, Harry is one of the main characters. There might also be his friends like Ron and Hermione. Since it's about being a founders heir, perhaps there will be appearances from the ghosts of the founders or characters who are descendants of the founders' friends or foes.
Godric Gryffindor is often very popular. He represents bravery, and in fanfiction, he can be the hero in many pre - Hogwarts stories. Writers like to explore his youth and how he came to be the great wizard he was in the original lore.
The heir of the founders in fanfiction could also be a character who is fated to bring back the original values of Hogwarts. Maybe in a story where the school has strayed from its roots, this heir emerges with the power to restore the balance between the four houses and the overall spirit of the school as it was intended by the founders.