Serena could be paired with them. She has her own dreams and skills in the Pokemon world. Her relationship with Ash could bring in some interesting romantic sub - plots. And then there's Eevee. Eevee is a very adaptable Pokemon. It could befriend Bonnie easily and help them in their adventures, like using its different evolution forms to solve various problems.
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.
Ash and his Pikachu would be great to pair them with. There's always been that rivalry between Team Rocket and Ash, and it could lead to some really fun and exciting interactions. Maybe in this fanfiction, they end up having to team up against a greater threat.
Another great character to pair them with could be Professor Oak. Imagine the chaos and hilarity when Jessie and James, the bumbling villains, interact with the wise and knowledgeable Professor Oak. He could be a mentor figure, guiding them towards a new path, or they could be constantly trying to outwit him for some reason.
Max is also a top character. His knowledge about Pokemon makes him a valuable asset in any story. He could use his smarts to figure out how to defeat a powerful enemy Pokemon or how to solve a mystery related to a certain Pokemon species. In many fanfictions, he is the one who comes up with the plans.
Misty would be a great character to pair with Coordinator Ash. Their long - standing friendship could add depth to the story. Misty could offer her own insights on Pokémon care and performance, having grown up in a Gym environment.
Blue could also be an interesting character to pair with Leaf. Their different personalities would create a lot of interesting dynamics. Blue's cocky attitude could contrast with Leaf's more calm and collected nature, leading to some fun interactions and perhaps even some friendly rivalries.
Brock would be an excellent character to include. He has been with Ash and Misty for a long time and his knowledge of Pokemon and their care adds a lot to the story. Also, Pikachu is a must - have. Pikachu is Ash's closest companion and its cute and powerful nature always makes for great interactions. Another good character could be Tracey. He can bring in his drawing skills and his unique perspective on Pokemon. His addition can create some interesting storylines, like him trying to draw a new and rare Pokemon that Ash and Misty encounter.
Pikachu is a must - include character. Since Pikachu has been with Ash from the start, it would add a sense of familiarity and also provide great comedic moments and powerful battle assistance. Brock could also be a good addition. His knowledge of Pokemon care and breeding could be useful in the story.
Definitely Team Rocket. Their constant attempts to steal Pokemon would create a lot of action and comedic moments. Nurse Joy is also important as she can heal their Pokemon when they are injured. Officer Jenny can be there to enforce the law when Team Rocket causes trouble. And Cilan from the Unova region. His expertise in Pokemon types and flavors could add a unique aspect to the story.
For Naruto, characters like Kakashi could be great to include. Kakashi's wisdom and experience would add depth to the story when combined with Naruto's energy. In Rossweisse's case, characters from her own world who have strong magical abilities could be paired with her. They could support her in her adventures with Naruto.