Voldemort can be a common character in such fanfics. If Harry dies, it might be at the hands of Voldemort or related to the fight against him. And Dumbledore, even though he might be dead in the original series, his influence and legacy could still be a part of the 'Harry Death Fanfic'. His wisdom might be something that the other characters reflect on after Harry's death.
Neville Longbottom could be a significant character. He has always been a strong and brave character. In a 'Harry Death Fanfic', he could step up in the absence of Harry. For example, if Harry dies before fully defeating Voldemort, Neville might take on the mantle and lead the fight, using the inspiration he got from Harry while he was alive.
Obviously, Harry would be one of the main characters. Since it's about him becoming death, he's at the center of it. There might also be his friends like Hermione and Ron. They could be reacting to Harry's transformation, either trying to help him or being afraid of the change in him.
Obviously, Harry is one of the main characters since he's in the title. But without reading it, it's hard to say who else might be in it. There could be characters from the original source material that the fanfic is based on, like his friends or enemies. Maybe there are also new characters created specifically for this fanfic that interact with Harry in his role as the angel of death.
Another theme is the exploration of life and death. These fanfics often look at the boundaries between the two. They might show Harry having the ability to see the souls of the dead, or being able to influence the passage of souls. It gives a different perspective on the afterlife in the Harry Potter universe.
Obviously, Harry and Tony Merchant are the main characters. Harry could be a heroic type, with special abilities or skills. Tony Merchant might be a more complex character, perhaps a bit of a rogue but with a good heart.
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.
There could be an antagonist in the fanfic. Maybe someone who had a vendetta against Harry Merchant and was somehow involved in his death. This character would add conflict to the story as the people left behind try to figure out what really happened to Harry.
Obviously, he could meet Darth Vader. Their encounter would be really interesting as they are from two very different fictional universes. He might also come across Luke Skywalker. They could form an alliance, with Harry sharing his knowledge of magic and Luke sharing his skills with the Force. Another character could be Han Solo. Harry would be amazed by Han's piloting skills and Han would be intrigued by Harry's magic.
Harry Potter himself is often a common character. Since it's a reincarnation fanfic, he could be reincarnated in different forms or with different personalities. Hermione Granger is also frequently seen as she is a very important character in the original series. Her intelligence can play a big role in the new storylines that the fanfic creates.
Dumbledore is a very common character. He is often depicted as having a grand plan to watch over Harry and protect him from Voldemort. Sirius Black is also popular. Fans like to write about him watching over Harry, especially since he was Harry's godfather. Hermione is another common one. Her intelligence and loyalty make her a great candidate for watching over Harry in fanfics.
The Weasley family, especially Molly or Arthur, can be common. They are nurturing figures and can provide a home - like environment for homeschooling. Molly might teach home - ec related magic like cooking spells, while Arthur could share his knowledge of Muggle - related wizardry.