Ash is of course a very common character as he is the central figure. Pikachu is also often included as it is Ash's most well - known Pokémon. Other characters like Misty and Brock might also make appearances as they are Ash's friends and travel companions in the original Pokémon series.
Since it's an 'ash the pokemon fanfiction', Ash is the central figure. But it could also feature his friends from the original series like Misty and Brock. And of course, the villains he encounters would be important characters too. There may be new characters created by the fanfiction writer, such as a mysterious trainer who has a rare and powerful Pokemon that Ash has to face.
The OC themselves are often popular characters. Since they are original creations, they can have all sorts of unique personalities, abilities, and backstories. For example, an OC could be a shy but brilliant Pokemon trainer who gradually overcomes their insecurities with Ash's help.
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.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
Ash is obviously one of the main characters. He is at the center of the harem concept. Then there would likely be a group of human characters who are part of his harem, though their specific identities can vary greatly depending on the fanfiction.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
Pikachu is always an interesting character in these fanfictions. In some stories, Pikachu might develop new abilities or have a deeper understanding of the Pokemon world that it can share with Ash. Another character could be Gary. He can be portrayed as a more complex rival, perhaps with his own hidden motives or a change of heart towards Ash. And then there's Misty. She can have new storylines in fanfictions, like being a powerful water - type Pokemon trainer with her own unique training methods that Ash can learn from.
In these stories, a good character to pair with Ash is May. May's adventurous spirit can match well with Ash's hybrid - enhanced adventures. She can be excited about exploring new places and Pokemon that Ash's hybrid nature might lead them to. Another one is Clemont. His scientific knowledge can be very useful in understanding Ash's hybrid physiology. He could be constantly trying to figure out how Ash's hybrid abilities work and come up with new strategies for battles. And Lyra from the Johto region can also be a great pairing. Her gentle nature can balance out any wild sides of Ash's hybrid self.