Hermione and Ron might also be involved. Hermione could help Harry in teaching the first - years, sharing her vast knowledge of magic. Ron could add his humor and loyalty to the group. And of course, Dumbledore or other Hogwarts professors might make appearances, either to support Harry or to provide some guidance on how to handle the young students.
Obviously, Harry Potter himself would be one of the main characters. And the first - years he's raising, though they might be a group of nameless first - years at first, some of them could emerge as more prominent characters as the story progresses.
Most likely the children of the main characters from the original series. Like James, Albus, and Lily Potter. Also, the kids of Ron and Hermione, like Rose and Hugo.
Harry Potter himself could be a main character. He has a history of standing up against injustice, so it's likely he'd be involved in a revolt. Hermione would also be a key figure. Her intelligence and knowledge of the wizarding world's laws would be invaluable in planning and leading a revolt. Ron, with his loyalty and courage, would be right there with them.
Harry Potter would probably be one of the main characters. He's the central figure in the original series, so it's likely he'd be involved in this fanfic in some capacity, perhaps as the hitman or someone related to the hitman - style plot.
Most likely, Harry Potter, Hermione Granger, and Ron Weasley will still be the main characters. After all, they are the central figures in the original series. But there could also be new characters introduced who are either part of their wealthy new circle or those who are affected by their millionaire status.
Obviously, Harry Potter himself would be one of the main characters. He's the hero of the original series and his story would likely continue in this fanfic.
In this fanfic, Harry Potter could be raising the first - years in a way that involves preparing them for the wizarding world outside of Hogwarts. He might introduce them to different magical creatures, not just the ones they study in class. He could also be mentoring them in how to deal with the politics and rivalries within the wizarding community. This would include teaching them how to handle encounters with Slytherins who may not always be friendly, and how to build bridges across house divides, just as he did during his time at Hogwarts.
Obviously, the illithid would be a main character. It could be a mysterious and powerful being that Harry encounters. Also, Hermione and Ron might still be important characters. They could be either suspicious of the illithid or try to help Harry understand it better. Dumbledore might also play a role, perhaps guiding Harry in his interactions with this strange new creature.
The main characters are Harry and Teddy, but don't forget about the wizarding community around them. There could be other minor characters such as Teddy's teachers at Hogwarts when he gets older. They would play a part in Teddy's development, and Harry would interact with them to ensure Teddy has a good educational experience.
Another possibility is that new characters might be created specifically for this fanfic. These characters could be hybrids of the two universes, having traits from both the Toaru series and Harry Potter. They could be students at Hogwarts who also possess esper abilities, or wizards who are involved in the affairs of the Toaru series' world. This would add a fresh and unique element to the fanfic.
Voldemort would remain a major character. Harry's new abilities and perspective from being raised by Death would make their conflict even more complex. Harry might approach the fight against Voldemort with a different strategy, one that is influenced by the teachings of Death.