There could be a wise old wizard who has knowledge about the true nature of the Master of Death and becomes Harry's mentor. Maybe a powerful magical creature that is drawn to Harry's power and offers its assistance.
Potential new characters in this fanfiction might include a seer who has foreseen Harry's rise as the Master of Death and tries to help him navigate his new role. There could be a magical blacksmith who creates special artifacts for Harry to help him control his power. Also, a mysterious figure from the past, perhaps a long - lost relative of one of the founders of Hogwarts, who has some connection to the concept of the Master of Death and emerges to offer their wisdom or interference.
Another potential set of new characters could be ancient wizards or witches who were once associated with the concept of the master of death. They could come back to either guide Harry or challenge his claim to the title. This would add a lot of depth to the story as Harry would have to prove himself to these predecessors or learn from their experiences.
There could be a dark arts teacher who is actually a double - agent. This character might seem to be teaching Harry the wrong things at first but is actually guiding him to use the Dark Arts for good. Another new character could be a mysterious figure from an ancient dark arts cult who tries to recruit Harry for their own purposes.
There could be an American wizard who takes Harry under his wing, like a gruff but kind old wizard named Alistair. He could teach Harry about American magic in a no - nonsense way.
There could be a female mentor who takes Ash under her wing because she sees potential in her as a girl in a male - dominated world. This mentor could teach her unique skills and ways of dealing with Pokémon. Another potential new character could be a rival who is also a girl and has a different view on how to compete in the Pokémon world compared to Ash. They could have some interesting battles and exchanges of ideas.
Maybe the Deathly Hallows have some sort of connection to Asgardian artifacts. Harry, in his role as the Master of Death, starts to investigate this connection and ends up in Thor's world. Thor initially doubts Harry but as they face challenges together, like an army of dark creatures from the wizarding world invading Asgard, they become friends and allies.
Perhaps Neville Longbottom could take on this role. He has shown great courage throughout the series. His experiences with loss, like the death of his parents, could lead him down a path where he becomes the apprentice of death, using his newfound powers to protect others and bring a new balance to the wizarding world.
Harry Potter himself could be a potential Avatar of Death. Given his connection to Voldemort and all the death and sacrifice he has witnessed, he might develop some sort of connection to death in a more profound way. It would be a very dark and different take on his character, but it could also add a whole new layer of depth.
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.
Well, Harry Potter is the key figure. In the harem, there could be characters like Daphne Greengrass. She is from a pure - blood family and could bring an interesting perspective. Pansy Parkinson could also be a part of it, which would create a lot of drama considering her previous attitude towards Harry. And then there's Tonks, who is a fun and unique character and could be an interesting addition to the harem.