Another possibility is an old wizard who has lived for centuries and has seen so much death that they have become obsessed with it. This wizard could be a hermit, living on the outskirts of the wizarding world. But one day, for some reason, they decide to enter the world again, specifically Hogwarts, because they believe that there is something there that will help them finally understand the true nature of death.
It could be a Slytherin student who has always been drawn to the darker aspects of magic. They might have grown up in a family with a history of dark magic, and this has influenced their view of death as something to be embraced rather than feared.
Well, it could be about a character who has an unusual fascination with death in the Harry Potter universe. Maybe they study dark magic related to death more closely than others. For example, they might be exploring the secrets of the Deathly Hallows in a very different way than what we see in the original books.
Harry Potter himself could be the main character. Since he has faced death so many times in the original series, it would be interesting to see how he interacts with the concept of Death from the Endless.
It could be a character who was once on the side of the light but was corrupted in some way and became Death's Knight. Maybe Neville Longbottom, who has always been underestimated. His transformation into Death's Knight could bring a whole new perspective to his character, as he battles his inner demons.
Perhaps an old follower of Voldemort who was in hiding. This person finds out about the Eyes of Death and believes that with their power, he can bring back Voldemort's legacy even stronger. He becomes the main force against Harry and his friends throughout the fanfiction.
It could be Voldemort again, but in a more powerful and death - centered form. Maybe he has found a way to fully embrace the power of death and become an even greater threat. Since Harry is the champion of death, it would be a battle of two opposing forces related to death.
Death, of course, would be a major character as it's training Harry. Maybe Dumbledore could also play an important role. He might be the one who initially guides Harry to this encounter with Death or acts as a moral compass during the training.
It could be an unknown character who has been in hiding all this time. Maybe she was once a powerful witch who made a pact with death and became its representative in the wizarding world.
Perhaps a dark wizard who is jealous of Harry's new power. This wizard could be someone who has long been in the shadows, waiting for the right moment to strike. He might see Harry as a threat to his own plans of taking over the wizarding world.
It could be Voldemort who somehow follows Harry into the Pokemon world. He might try to use the power of Pokemon to regain his strength and take over both the wizarding world and the Pokemon world. His dark magic could corrupt some of the Pokemon, turning them into his minions.
It could be Voldemort who somehow follows Harry into the DC universe and teams up with some of the DC villains like Lex Luthor. Their combined evil intellect and power could pose a great threat to both Harry and the DC heroes.