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.
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.
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.
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.
Naruto is definitely one of the main characters. He is the key figure in the Naruto universe. His family, which includes Hinata, Boruto, and Himawari, are also likely to be main characters. Boruto, with his own dreams and potential, might be facing some internal or external conflicts. Himawari, being the younger one, could be used to show the family's protective side. And of course, other characters from the original series like Sasuke might also make appearances, as he is Naruto's best friend and they have a deep connection.
Obviously Naruto himself. Then Sasuke would probably be involved as they have a deep connection. Sakura too, since she's part of their team. Kakashi might also play a role as Naruto's former teacher.
Well, Mumen Rider could potentially be in a comical sense in a One Punch Man harem fanfiction. Just kidding! But seriously, characters like Fubuki are prime candidates. She has a strong personality and her own motives which could easily be intertwined with romantic feelings for Saitama. Also, any new female characters introduced to the One Punch Man universe in the fanfiction could be part of the harem. They might be normal civilians who witness Saitama's heroics up close and become smitten.
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.