Espeon's presence could create a bit of competition within Ash's team. Some Pokémon might feel the need to prove themselves against this new and powerful addition. However, over time, they would all realize that they are stronger together. Espeon could also help Ash better understand the individual strengths and weaknesses of each Pokémon on his team, allowing for more customized training regimens.
Since Espeon is often seen as an elegant and intelligent Pokémon, it might influence the behavior of Ash's more rambunctious Pokémon. It could teach them to be more strategic and calm. On the other hand, the other Pokémon might also introduce Espeon to a more fun - loving and adventurous side of life, changing Espeon's somewhat reserved nature a bit.
The plot could involve Ash stumbling upon an Espeon that has lost its memory. Ash tries to help it regain its memories. Along the process, Espeon starts to remember bits and pieces which lead them to different locations. These places hold secrets and also some powerful Pokémon. Ash and Espeon team up with his other Pokémon to uncover the secrets and protect the area from villains.
To write a compelling Espeon character, first focus on its psychic abilities. Describe how it uses telepathy to communicate with other Pokemon and its trainer. For example, it could send thoughts to its trainer during battles to suggest strategies. Also, give it a distinct personality. Maybe it's a bit mischievous and likes to play pranks on other Pokemon using its powers. Another aspect is its loyalty. Show how it would do anything to protect its trainer or its friends.
It would likely bring a new level of harmony (literally) to Ash's team. Since Meloetta has musical powers, it could use its melodies to boost the morale of the other Pokémon on the team. For example, before a big battle, Meloetta could sing a song that makes Pikachu feel more energetic and ready to fight.
Growlithe could be a very loyal addition to Ash's team. It might have a more protective nature compared to some of Ash's other Pokemon. This could change the way Ash approaches battles. Instead of always going on the offensive, he might rely on Growlithe to defend the team first. Also, the other Pokemon on the team might look up to Growlithe's courage, which could inspire them to be braver in battles.
Well, a great storyline might be Espeon getting involved in a time - traveling adventure. It could end up in a different era of the Pokemon world and have to adapt. Maybe it meets ancient Pokemon and has to use its talking ability to negotiate with them. Also, a story where Espeon is on a quest to find a rare crystal that enhances its psychic powers. Along the way, it meets various challenges and makes new friends. And then there's the idea of Espeon being part of a Pokemon detective story, using its intelligence and talking skills to solve Pokemon - related crimes.
Well, it could be due to a psychic bond. Suppose there's a strong psychic Pokemon or a powerful psychic trainer in the fanfiction. They might form a connection with Biy. Since Espeon is a psychic - type Pokemon, this bond could gradually change Biy's form into that of Espeon. The psychic energy shared between them could be the catalyst for this transformation, which would make for an interesting story arc.
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.
In the fanfiction, Biy turning into Espeon could mean a shift in its place in the Pokemon ecosystem. Espeon is a more evolved and powerful form. So, it might be targeted by stronger predators or rival Pokemon. Socially, among other Pokemon, it could face isolation as it is no longer the same as before. Also, its diet might change, which could affect the availability of food resources in the area where it lives.
He might become more cautious. Usually, Ash is quite bold and rushes into things. But if he's in a Team Rocket - centered fanfiction, he'd likely be more careful as he's constantly surrounded by danger. For example, he'd think twice before challenging a new Pokemon or entering an unknown area.
Pikachu would likely be very confused and resistant. Pikachu has a strong bond with Ash and knows his true nature. It would probably try to shock Ash out of whatever trance or false belief he has from joining Team Galactic. For example, when Ash starts to follow Team Galactic's orders that go against his usual principles, Pikachu might refuse to cooperate and try to show Ash that he's on the wrong path.