Obviously, Harry Potter is the main character as it's centered around him being the Immortal Master of Death. He would be a more powerful and perhaps more complex version of himself from the original series.
Hermione Granger could also be a significant character. She might be by Harry's side, helping him understand the implications of his new state. She could use her vast knowledge to research more about the Master of Death concept and assist Harry in controlling his powers.
Ron Weasley could play an important role too. His loyalty to Harry would be tested in new ways as Harry's immortality and power change their relationship dynamics. He might be initially in awe of Harry but then have to deal with his own insecurities as Harry becomes something more than just a friend.
Ron Weasley could be another main character. He might be by Harry's side, providing the loyalty and friendship he always has. In some fanfictions, he could also gain some unique abilities or knowledge related to Harry's new state. And Dumbledore, even in death, might still have an influence. His past knowledge and wisdom could be a guiding force for Harry as he navigates his new immortality and power as the Master of Death.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
Obviously, Harry Potter himself. He's the one who has become the Master of Death. Hermione and Ron would also be important. They'd be there to support Harry, and also to question his new power and the moral implications. Dumbledore might also play a role, perhaps as a guide from beyond the grave, since he was always so interested in the Deathly Hallows and what it means to be the Master of Death.
Harry Potter is a given. In addition to him, I think there could be other characters from the Harry Potter series who might also make an appearance. From the Charmed side, apart from the Halliwell sisters, characters like Leo Wyatt could be important. Since it's a fanfiction that combines two universes, it might also introduce new characters that are created specifically for this story to bridge the gap between the two worlds. For example, a character who has knowledge of both the magic in the Harry Potter world and the power of the Charmed ones.
Obviously, Harry Potter is the main character as it's about him being the 'Son of Death'. His friends Hermione and Ron are likely to be important characters as well. They always play significant roles in Harry's life. Dumbledore might also be a key character, perhaps guiding Harry in understanding his new identity.
Hermione would surely be one of the main characters interacting with Harry. She would be both intrigued and cautious about his new status. She might help him research and understand the full implications of being the master of death.
It depends on the specific fanfic. But it could be Harry Potter himself in a new role as a puppet master - like figure. Or perhaps a new character created by the fanfic author who has the powers of a puppet master and interacts with the well - known Harry Potter characters.
It could also be that an existing character from the Harry Potter series discovers or develops this new power. Maybe Hermione, with her brilliant mind, somehow unlocks this ability and has to deal with the consequences. She would have to balance her new power with her existing knowledge and friendships.
Ron Weasley could be another main character. He would be by Harry's side, providing support and his own brand of wizarding skills. Sometimes, characters like Dumbledore might also play a significant role. Even though he's dead, his wisdom could be passed on to Harry in some way, guiding him as he navigates his role as the Master of Death.
Voldemort would remain a major character. Harry's new abilities and perspective from being raised by Death would make their conflict even more complex. Harry might approach the fight against Voldemort with a different strategy, one that is influenced by the teachings of Death.