In this fanfiction, Harry is the main character as the Ice King of Slytherin. Other important characters could include the other Slytherin students. For example, Pansy Parkinson might be a character who initially mocks Harry but then comes to respect him. There could also be older Slytherin alumni who come back to the school when they hear about the new Ice King. And Dumbledore would still be a key figure, observing Harry's journey and offering his wisdom and guidance from the sidelines.
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.
Obviously, Harry Potter himself is crucial. Then there would be the other Slytherin students like Draco Malfoy. Their relationship could be very different in this fanfiction. Maybe they become allies instead of rivals. There could also be the Slytherin professors who play important roles in guiding Harry as the new 'king' of the house.
Snape himself could also be considered as the 'Prince of Slytherin' in some fanfictions. Given his complex relationship with both Slytherin house and Harry, a fanfiction could explore a scenario where he is given this title due to his deep connection to Slytherin's heritage and his own role as a double agent, which in a way makes him a sort of prince - like figure within the Slytherin world.
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.
The plot might involve Harry being magically transformed into the Ice King of Slytherin against his will. He struggles to control his new powers which are linked to ice and cold. In Slytherin, he has to face the prejudices of his housemates who initially don't trust him with this newfound power. But as he learns to master it, he uses it to protect Slytherin and Hogwarts from a common enemy.
Obviously, Harry is a main character as he is the king of Slytherin in this fanfiction. Hermione might also be involved, providing her intelligence and support to Harry in his new role.
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.
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 because some fans want to explore a different side of Harry. By making him the 'king' of Slytherin, they can show how his character could adapt and thrive in a different house's environment.
Draco Malfoy is often a central character. His complex personality and his Slytherin pride make him a natural fit in these types of fanfictions. He can be both a love interest and a source of conflict.
The Slytherin Prince is, of course, central. Then there might be characters like Draco Malfoy, who could be either a friend or a rival to the Slytherin Prince. Hermione Granger could also be involved, as she often crosses paths with different characters in fanfictions. Additionally, there could be original characters created specifically for this fanfiction, perhaps a long - lost relative of the Slytherin Prince who has some secrets to share.