For the Atlantis side, there could be Atlantean royalty, like a princess or a group of elders. There might also be Atlantean mages or warriors. And don't forget about the mer - people. They could play a role, either as allies or antagonists. Maybe there's a mermaid who falls in love with Harry and helps him understand the ways of Atlantis.
Obviously, Harry Potter himself as the Ice King. Then there would be Draco Malfoy. He could be either a rival or an ally to Harry in this new role. Hermione would also be there, trying to understand the magic behind Harry's transformation and helping him control his powers. And of course, Snape, who might have some knowledge about the Slytherin magic related to the Ice King.
In this fanfiction, Voldemort could also be involved in a different way. Since Harry is now the son of the Demon King, Voldemort might see him as a threat or a potential ally. Dumbledore would surely play a role too. He might be the one to guide Harry or try to keep him from fully embracing his demonic side. And there could be some magical creatures that are associated with the demonic realm, like imps or minor demons that could be characters in their own right.
There could be a wise old wizard or witch who acts as a mentor to the heir of Atlantis. This character would have knowledge about Atlantis and its secrets, and would guide the heir in understanding their heritage and powers. Another possible main character could be a villain who is trying to stop the heir from fulfilling their destiny, perhaps because they want to claim the power of Atlantis for themselves.
Obviously, Harry Potter would be one of the main characters. And there would probably be some Atlantian characters like the King or Queen of Atlantis, or other important Atlantian figures. Maybe some of Harry's friends from Hogwarts could also be involved, like Hermione and Ron, if they also get dragged into the Atlantis adventure.
Definitely Harry Potter, who would be curious about this new development in the wizarding world. Neville Longbottom could also play a role, maybe he has some knowledge about the old dynasties that could help or hinder the king's plans. The Weasley family as a whole could be involved too. Ginny Weasley might have her own take on the situation. And there could be some ancient wizards who were thought to be long dead but are awakened with the king's return. These characters could add a lot of depth and mystery to the fanfiction.
Maybe Harry discovers he has some hidden connection to Atlantis. He could be magically transported there and due to his powerful magic and heroic nature, he ends up becoming the king. He then has to deal with Atlantis's internal politics and protect it from external threats.
One possible plot is that Voldemort also has an interest in Atlantis. Harry, as the King of Atlantis, has to not only protect his new kingdom but also prevent Voldemort from getting his hands on Atlantean power sources. This could lead to epic battles underwater, with Harry using his Patronus and other spells against Voldemort's dark magic, all while trying to uphold the traditions and values of Atlantis.
Obviously, Harry Potter himself would be the central character. Then, Hermione Granger could be one of the members of the harem. She is intelligent and has a long - standing friendship with Harry. Ginny Weasley is also a likely candidate as she has a romantic interest in Harry in the original series. Luna Lovegood might be another, with her unique and dreamy personality adding an interesting dynamic to the harem.
A less obvious candidate could be a corrupted Dumbledore. Imagine if he was swayed by the allure of absolute power and became the dark king. It would be a huge twist as he is seen as a symbol of good in the original books.
The Death Eaters could still be a major threat. Even with Voldemort gone, some of them might be trying to revive the dark ways and see the wizard king as a big obstacle in their path.