Well, Percy Jackson is most likely one of the main characters, given that his name is in the title. And since it's related to the Percy Jackson universe, Annabeth Chase could also be a main character. She often plays a significant role in Percy's adventures. Also, if the 'Heir of Black' is a new character, that would be a main character too.
In the 'Percy Jackson Heir of Black' fanfiction, Percy Jackson is the obvious main character. Then, depending on the plot, the 'Heir of Black' character, whether it's a human, demigod or some other mythical being, would be a main character. There might also be antagonists who are trying to stop Percy from fulfilling whatever role is associated with being the 'Heir of Black'. These antagonists could also be considered main characters in terms of driving the plot forward through their actions against Percy.
The heir of Pontus is the central figure. They will have their own personality, goals, and struggles. Alongside them, if the fanfiction is set in the Percy Jackson universe, characters like Grover could make an appearance. Grover might be involved in guiding the heir in some way, or he could be a sidekick during their adventures. Also, there could be villains in the form of other powerful beings from the sea or those who want to take advantage of the heir's power. For example, a sea witch - type character who tries to manipulate the heir.
Obviously, Percy Jackson is one of the main characters as he is the son and heir of chaos in this fanfiction. His friends from the original series like Annabeth Chase could also be main characters. They would likely play important roles in supporting Percy on his new journey and dealing with the implications of his new status.
The 'Percy Jackson Heir of Black' fanfiction could potentially revolve around Percy being thrust into a situation where he has to deal with the implications of being associated with the 'Heir of Black'. It could be that there are ancient prophecies or secrets that tie him to this title. It might include battles against new enemies who are interested in either protecting or destroying the 'Heir of Black' and Percy gets caught in the middle. It could also explore how Percy's friends, like Annabeth, Grover, etc., react to this new development in his life.
Percy Jackson himself is obviously very popular as he is the main character. His friends like Annabeth Chase also tend to be popular as readers are interested in how their relationships develop. Grover Underwood is another one that fans like to see in these fanfictions, especially when he uses his satyr abilities to help Percy on his adventures as the heir of chaos.
Draco Malfoy would be a likely character. Given his Slytherin status, he would likely have a complex relationship with Percy. He might start off as hostile, seeing Percy as an outsider, but then they could end up having to work together for some reason.
Well, the main antagonists could be a group of rogue demigods who have formed a cult around Kronos. They believe that Percy, as the heir of Kronos, should lead them in overthrowing the current order of the demigod world. But Percy doesn't want any part of it, so they become his enemies. Another possible antagonist could be an ancient being that was once in league with Kronos and now sees Percy as a means to resurrect Kronos or continue his dark legacy. This being would stop at nothing to force Percy to embrace his role as the heir.
A primordial being, like Chaos or Nyx, could also be the main antagonist. If they feel that the Olympians have become too powerful or are misusing their power, they might decide to wipe out the entire demigod race. Percy would then have to find a way to either convince them to spare the demigods or find a way to defeat them, which would be extremely difficult as they are among the most powerful entities in Greek mythology.
Well, obviously the black heir is one of the main characters. Their actions and decisions drive a lot of the story.
In most Percy Jackson canon, there is no relation to Thor. However, in fanfiction, anything can be possible. It could be that the author has created a unique crossover where Percy Jackson has some connection to being the heir of Thor, perhaps through some magical or divine intervention that the fanfic writer has dreamed up.
Obviously, Harry Potter is one of the main characters since the story is centered around him being the heir of Black. Sirius Black might also be an important character, as he was a Black himself and could play a role in Harry's discovery or training. And of course, Harry's friends like Hermione and Ron would likely be involved too.