Obviously, the son of Wonder Woman is a main character. His character development would be central to the story. He could be named something like Alexander or something more unique.
Wonder Woman herself would also be an important character, even if she's not the main focus. Her influence on her son and her presence in his life would shape the story. She might pass on her wisdom and values to him.
There could also be other supporting characters. For example, Batman might be a character in the fanfiction. He could be a bit skeptical about their marriage at first, worried about how it will affect the balance of power in the superhero world. Another possible character could be Lois Lane. Her reaction to Superman marrying Wonder Woman could add an interesting dynamic, like she might be a bit jealous but also come to accept it in the end.
The main antagonists could be the Titans who see Percy as a threat because of his combined powers. They might try to stop him from fully realizing his potential as the son of Wonder Woman.
There could be an ancient evil that is awakened by the birth of their son. This entity could see the son as a threat to its plans for world domination. It might have powers that are beyond what the son initially thinks he can handle, but through his own growth and the help of his parents' friends, he is able to overcome this powerful antagonist.
Wonder Woman is surely one of the main characters. And Steve Trevor might also be a main character as he has an important role in Wonder Woman's story.
Obviously, Wonder Woman herself is the central character. She is a powerful and inspiring figure. Then there's Steve Trevor, who often plays an important role in her stories. He is her love interest and also helps her understand the world of humans.
In the 'Wonder Woman 600 Story', Wonder Woman is the central figure. Alongside her, Steve Trevor, her love interest and a key ally in her adventures in the mortal world. The Amazons play a crucial role too. Their queen, Hippolyta, may be involved in guiding Wonder Woman or in dealing with threats to the Amazonian island. Also, there could be some of her classic villains like Cheetah or Ares who make an appearance as main antagonists, trying to disrupt Wonder Woman's mission of peace and justice.
The main character is without a doubt Diana, the Wonder Woman. She has her own set of powers and a strong sense of justice. Steve Trevor is another important character. He is the one who gets Diana involved in the affairs of the outside world. Queen Hippolyta also matters as she is Diana's mother and the ruler of Themyscira. There are also some villains like Ares, the God of War, who is a key figure as Diana believes he is causing the chaos in the world.
Darkseid is another potential foe. He could be interested in the son's unique heritage, wanting to either corrupt him or use his powers for his own evil purposes. Given Darkseid's power, the son would have to rely on both his Batman - like wits and Wonder Woman - like strength to oppose him.
The main character is surely Wonder Woman. Regarding other main characters, it's difficult to say precisely without more knowledge of the novel. However, if it follows the 'Hiketeia' theme closely, there could be a character who represents the other side of the 'Hiketeia' relationship. For example, a character who has sworn loyalty to Wonder Woman in the context of 'Hiketeia' or a character who has put Wonder Woman in a difficult position regarding her 'Hiketeia' - related obligations.
The magical and superhero elements could blend in strange and exciting ways. For example, Wonder Woman's lasso of truth could have an interaction with the magic in the Harry Potter universe. It could be used to force dark wizards to tell the truth, or it could have some sort of magical property when combined with spells. Also, Harry's relationships with other characters would change. Hermione and Ron might be in awe of his new powers and his connection to a superhero, and they could all go on adventures that are not just about defeating Voldemort but also about protecting the world in a more global sense, including the Muggle world which Wonder Woman is also committed to protecting.