The main character is Harry Potter without a doubt. However, new characters can also be introduced in these fanfictions. For example, there could be a local wizard or witch in the place where Harry is exiled who becomes his friend or mentor. This new character can have their own backstory and motives. They could teach Harry about the local magic or help him adapt to his new environment, adding a new dimension to the story.
Another character that might be involved is Bellatrix Lestrange. As a devout follower of Voldemort and a member of the Black family, she could have complex relationships with Lady Black. Maybe in some fanfictions, Lady Black has to deal with Bellatrix's extreme actions and how they reflect on the family name.
Hermione could also be a main character. She is known for her intelligence and research skills. In a fanfiction, she could be the one researching the history of the snake staff, trying to understand its true nature and powers, and using her knowledge to either protect it from falling into the wrong hands or to unlock its potential for good.
Well, aside from the obvious ones like Harry and Hermione, Luna Lovegood can be a main character in some 'Harry Potter Master Legilimency' fanfictions. Her unique perspective on magic could lead to some interesting takes on Legilimency. Sirius Black is also a character that might be involved. He was a member of the Marauders and could have passed on some knowledge about Legilimency. And then there's Dumbledore. Given his vast knowledge and power, he could be guiding the main characters in their Legilimency studies or using it in his grand plans for the wizarding world.
Hermione is often a main character in these fanfictions. Given her love for learning and thirst for knowledge, it's easy to imagine her exploring the intersection of science and magic.
Harry Potter himself is often involved. He might be the one to encounter the first signs of inflation, whether it's a magically inflating potion or an enchanted object. His friends, like Ron and Hermione, are also likely to be part of the story, helping him figure out what's going on.
Ron Weasley could also be a key figure. He might be Harry's loyal friend as always, standing by his side during all the changes that Harry is bringing about in the wizarding world as a god. And there could be new characters introduced as well, perhaps other gods or magical beings that interact with Harry in this new fanfiction realm.
In some cases, Albus Dumbledore could play an important role. His wisdom and power in the wizarding world would make him a key figure in any situation where the two different universes collide. He might be the one to first discover Alexstrasza's presence and figure out how to make use of her abilities in the fight against the dark forces in the Harry Potter world.
Obviously, Harry Potter himself is always a main character. He is the one most directly associated with the concept of death in the original series, what with his parents' death and his multiple near - death experiences.
Ron Weasley could be another main character. He might be initially skeptical about the piano's magic but then be amazed when he sees the extraordinary things it can do. Maybe he accidentally discovers a new function of the piano while fooling around with it in the Gryffindor common room.
Harry Potter himself could be one of the main characters in such fanfiction. Maybe he gets an automail limb after a particularly dangerous encounter with Voldemort. Hermione might also be involved, using her vast knowledge to help understand and improve the automail technology within the wizarding context.