Definitely Castiel. Castiel has his own complex relationship with death and the divine in the Supernatural world. Pairing him with the Angel of Death would create a really interesting dynamic. They both have different views on mortality and the afterlife, which could lead to some great character interactions.
I don't know as I haven't read it. But it might have the angel of death as a main character. There could also be characters who interact with the angel, like the ones whose lives are affected by it, such as the people it is sent to or those who try to stop it.
Without knowing the particular 'angel of death novel', we can only speculate about the main characters. The 'angel of death' would surely be a central figure. There could be a human character who is on a journey to understand the nature of death and in the process crosses paths with the 'angel of death'. Another possible main character could be a religious figure who believes in the concept of the 'angel of death' and tries to either communicate with it or prevent it from carrying out its deeds. And there might be a character who has been marked for death by the 'angel of death' and is fighting for their survival.
There might be a human character who somehow gets involved with the angel of death. This human character could be on a quest or facing a personal crisis that intersects with the angel of death's existence. Maybe there is also a sidekick or a mentor - like figure that helps the main character navigate the strange world of the story.
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.
Well, apart from Ib and the Angel of Death, there could be other characters. Maybe there are some other lost souls in the world that Ib and the Angel of Death encounter. These could be minor characters who either help or hinder Ib's progress. But the central focus would still be on the relationship and interactions between Ib and the Angel of Death.
Well, Percy Jackson is obviously one of the main characters. He's the hero of the original series, so in the fanfiction related to the Angel of Death, he's likely to be in the center of the action. The Angel of Death itself is another main character. It could be a new, original creation for the fanfiction or a re - imagining of an existing mythological figure. And there might also be Percy's friends like Annabeth. She could play an important role, maybe trying to stop Percy from getting involved with the dangerous Angel of Death or helping him understand the situation better.
The Woman in Black is a central character in the story. She is a malevolent spirit who brings death and fear. The young teacher, Eve, is also a main character. She is kind - hearted but gets caught up in the horror of the Woman in Black. Then there are the children at the orphanage. They are important as they are the ones being haunted by the Woman in Black. Each child has their own personality, and their innocence is in contrast to the evil that surrounds them.
There are also stories in disaster situations. For instance, during a major earthquake, a rescue worker might have to make the difficult call to end the life of someone who is trapped and in extreme pain with no hope of rescue. While it's a very tough decision, it can be seen as a mercy in the long run. It's a complex situation that blurs the lines between what we might consider a negative 'Angel of Death' and a more positive, if still very difficult, act.
There are also stories from war zones. Sometimes, in the chaos of war, there are individuals who take on the role of the 'Angel of Death' in a more ambiguous way. For example, a medic might have to make difficult decisions about who to save and who not to save due to limited resources. This can seem like a form of the 'Angel of Death' concept, though it's a very different context from a criminal like Shipman.