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 might involve the Deathly Hallows taking on new significance in relation to the Endless Death. For example, the Elder Wand could be seen as a tool that even Death from the Endless might be interested in. The Resurrection Stone could have a connection to bringing back those who have passed on in a way that goes beyond the normal magic in Harry Potter, influenced by the nature of Death in the other fictional context.
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.
Harry would likely develop a very different personality. He may be more solemn and less prone to the typical teenage angst that the original Harry had. Since he is raised by Death, he might be more in tune with the darker aspects of life and magic from a young age.
Hermione could be a main character. She might be the one who first notices the strange changes in Harry after he becomes friends with Death. She could try to research about this unusual friendship in the library, using her vast knowledge to understand the implications. Ron could also be important. He might be initially skeptical about Harry's new 'friend' but then come to accept it as he sees the positive effects it has on Harry and the help it provides in their fight against dark forces.
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.
Yes, there are likely fanfictions out there with this concept. The 'Harry Potter' fandom is vast and creative, and many fans like to explore unique and alternative storylines. Some might imagine Harry having a special connection to the idea of 'Lord Death' in a fan - created narrative.
Obviously, Harry Potter is one of the main characters. And Death, which is given a more prominent role in this fanfiction compared to the original books. There may also be other characters from the original series like Hermione and Ron who might play important roles in the story, perhaps helping Harry in his interactions with Death or being involved in the events that are related to the new plotlines involving Death.
The main characters would surely include Harry Potter. He is the one making the trade with Death. Death is also a main character as it is the entity with which the trade is being made. Additionally, Dumbledore could potentially play a role, either as a guiding spirit from beyond (if he has passed away in the context of this fanfiction) or as someone whose teachings influence Harry's decision to make the trade.
Harry Potter would definitely be one of the main characters, since it's about him being the 'child of death'. There might also be his friends like Hermione and Ron, as they are often involved in his adventures.
Another theme is adventure. The characters from Harry Potter are often thrust into the unknown of the Death Star environment, which leads to all kinds of exciting and dangerous situations that they have to navigate through using their wits and magic.