His other Pokémon like Rowlet might be in awe. Rowlet is a curious Pokémon, and having a mythical Pokémon like Tapu Koko around would be fascinating. It could learn a lot from Tapu Koko, especially in terms of using its unique powers more effectively. The whole team would likely feel a sense of pride that Ash was able to catch such a rare and powerful Pokémon.
Some of Ash's more competitive Pokémon, like Infernape (if it were still with him), might see Tapu Koko as a great addition to the team for battle purposes. They would look forward to training with it and learning new strategies. However, the more gentle Pokémon on the team, like Lilligant, would be happy just to have a new friend around and would enjoy the positive energy that Tapu Koko would bring to the group.
In the fanfiction, Ash could be on a special quest to catch Tapu Koko. It could be related to saving the Alola region from some kind of threat. So, catching Tapu Koko becomes not just a personal achievement but also a way to protect the land and its people. He might need to prove himself worthy to Tapu Koko before it allows itself to be caught.
Well, for one, Haunter has some really cool moves like Hypnosis and Dream Eater. When Ash catches it, his team's battle strategies would change. His other Pokémon would need to learn to work with Haunter. Also, Haunter's presence might intimidate some of the opposing Pokémon, giving Ash an edge in battles. It could also open up new exploration opportunities as Haunter might be able to access areas that other Pokémon on his team can't due to its Ghost - type nature.
Some social media groups dedicated to Pokemon also might be a good place to look. People in these groups may have written or know of some fanfictions about Ash catching Tapu Koko. For example, Pokemon fan groups on Facebook or Reddit.
One possible plotline could be that Ash meets Tapu Koko in a mysterious forest. Tapu Koko is initially cautious of Ash, but Ash shows his pure love for Pokémon through his actions like sharing his food with other Pokémon. This earns Tapu Koko's trust gradually, and eventually Ash manages to catch it in a very unique Poké Ball. It could be a special one given to him by an elder in the Alola region.
One common theme is friendship. Writers often explore the special bond that can form between Ash and Tapu Koko. Since Tapu Koko is a powerful and somewhat mysterious Pokémon, the idea of it befriending Ash is really appealing.
His Pokémon would probably be very confused at first. Pikachu, for example, might try to zap Ash back to his senses if he starts acting like a Team Rocket member. They would likely sense that something is wrong with Ash and would do their best to get him away from Team Rocket's influence.
The Pokémon League might condemn Ash's actions. Despite Team Rocket's wrongdoings, killing is not an acceptable solution in the Pokémon world which values friendship and co - existence. Ash could face consequences such as being banned from Pokémon battles or certain areas. On the other hand, some trainers who have been harassed by Team Rocket for a long time might secretly support Ash.
One common theme is friendship. Ash and Tapu Koko form a unique bond that often helps them face challenges. Another theme is adventure. They usually embark on some exciting journey together in the Alola region or beyond. For example, in many fanfictions, they might explore a hidden island or a secret part of the forest.
Pikachu would probably be very curious. It might zap the lightsaber a bit to see what would happen, and then be in awe of the bright blade. It could also get a bit protective of Ash when he wields the lightsaber, as if it senses the added danger.
When Ash catches Greninja, it might change the dynamics within his team. Greninja could be a bit of a loner at first, not really fitting in with the rest of the Pokémon. But over time, through shared battles and experiences, it could learn to work well with the others and even inspire them to improve.