Obviously, Ash would be the main character. Then there could be female characters who form his harem. Maybe some of his long - time friends like Misty and Brock could also be involved in some way, even if they are not part of the harem directly. There might also be new characters that he meets during his disappearance.
Well, it could be that Ash disappears on a dangerous quest. His friends in the harem are distraught. When he returns, he might have new powers. And then he has to deal with the changed dynamics in the harem, like some might be angry at him for leaving, while others are just overjoyed to have him back.
Definitely Pikachu. It's Ash's closest companion, so it could be part of the harem in a very unique way. Then there could be female Pokémon trainers like Misty and May. They have a history with Ash and could be part of this harem concept.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
The setting in which Harry disappears and returns also matters. If it's in the middle of Hogwarts, the school would be in chaos. Teachers would be trying to keep the students calm while secretly searching for Harry. And when he returns, the reactions of different houses - Gryffindor being relieved and proud, Slytherin perhaps being suspicious - could add more layers to the story.
Obviously, Alexis and Castle would be the main characters. Their relationship as father and daughter is central to the 'Castle' series, so in this fanfic, their dynamic would be crucial. Castle would be the one trying to find Alexis, and Alexis would be the one in the middle of whatever situation led to her disappearance.
Gary Oak is a likely character. He has always been a rival to Ash and in fanfiction, he might betray Ash out of jealousy or a desire to be the ultimate Pokémon trainer. Another could be Misty. Maybe she gets influenced by some external force or her own insecurities and turns against Ash. Brock could also be a candidate in some stories. If he falls in love with a girl who has a vendetta against Ash, he might be forced to choose between his loyalty to Ash and his new love interest.
In certain fanfictions, a Pokémon professor or an authority figure in the Pokémon world could betray Ash. They may have their own secret agendas. For example, a professor might use Ash's research data without his permission or manipulate him into dangerous situations for their own experiments. This would make Ash feel extremely betrayed as he looks up to these figures for guidance.
In fanfiction, Gary could be a likely character to betray Ash. He has always been a rival and might have ulterior motives. For Dawn, Zoey could potentially betray her. Zoey has been a tough competitor and might be jealous enough to take some underhanded actions. Another possible betrayer for Ash could be Team Rocket. They've been after his Pikachu for a long time and could use betrayal as a new tactic. For Dawn, perhaps a new coordinator who is envious of her popularity in the coordinator world.
Team Rocket is a likely candidate. They're always up to no good and hypnotizing Ash would be just another devious plan for them. They could use it to finally capture Pikachu. Giovanni, as their leader, might also be involved in such a scheme. Another character could be a powerful and envious trainer who wants to eliminate Ash as competition by hypnotizing him and making him lose his skills.
Obviously, Ash and Greninja are the main characters. Ash, the trainer with a big heart and a lot of determination. Greninja, the powerful and unique Pokémon with its cool design and amazing abilities.