On the Hogwarts side, Hermione and Ron would surely be there. They could be quite bewildered by Harry's newfound connection to Olympus. And Dumbledore, he might have some knowledge about the connection between the two worlds and could guide Harry in some way. From the Olympus side, Athena could also be involved as she is the goddess of wisdom and might see potential in Harry.
Obviously, Harry Potter himself. Then there could be Hermione and Ron, who would be his friends and help him deal with the revelations of his parentage. Dumbledore would also be there, guiding Harry as he tries to understand his connection to Ariel. And perhaps some sea - based characters like mermaids or sea wizards who come to Hogwarts to interact with Harry.
There could also be new characters introduced. Maybe other magizoologists from different parts of the wizarding world. They could have different approaches and beliefs about magical creatures. Some might be rivals, trying to outdo Harry in the discovery and study of new species, while others could be mentors, guiding him in the complex world of magizoology.
Well, in this type of fanfiction, Hermione could also be an important character, but in a different light. Maybe she tries to stop an evil Harry, or perhaps she is corrupted by him and becomes his accomplice. Other characters like the White Witch from Narnia could be involved. Her icy magic and cold - hearted nature could be an interesting contrast or complement to an evil Harry's powers. And don't forget about characters from other YA dark fantasy series, such as the Darkling from Shadow and Bone, who could potentially cross paths with an evil Harry.
Dumbledore could make an appearance. He might be the one who gives Harry the idea to become a pilot in the first place or provides some ancient magical knowledge about flying that helps Harry in his new career. Also, the Weasley twins could be involved. They could create some magical gadgets for the plane, like a device that makes the plane do cool stunts.
The Weasley twins could be part of it too. Suppose Harry yells in excitement or anger about a new prank idea that the twins have come up with that might be a bit too dangerous. They could either encourage his yelling or try to tone it down, depending on their mood. And Malfoy might overhear Harry's yell and use it as an opportunity to taunt him, adding more conflict to the fanfiction.
The Avengers could be involved. Captain America, for instance, might be suspicious of this 'Loki' (who is actually Harry) and keep a close eye on him. Iron Man could use his technology to analyze Harry, thinking he is Loki. And from the Harry Potter universe, Ron could be there, trying to support Harry in this strange situation where he is mistaken for someone else entirely.
Hermione would definitely be there. She would be intrigued by Harry's new heritage and would probably try to research all about Ra in the Hogwarts library. Ron too, would be by Harry's side, though he might be a bit more skeptical at first about this whole Ra business.
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.
Obviously, Harry himself as the son of Atlas. Then there could be the original characters like Dumbledore, who might play a role in guiding Harry about his new identity. Hermione would still be there, probably helping Harry research about Atlas. And Ron, as Harry's friend, would be involved in his adventures.
Obviously, Harry Potter himself. Then there could be the dragons, perhaps an old and wise dragon that acts as Harry's mentor. Hagrid might also play an important role as he has a connection with magical creatures and could help Harry understand his new identity.