Mewtwo could be an interesting addition. Given its origin as a genetically - engineered Pokemon, it can bring a whole new level of complexity to the fanfic. It's extremely powerful and its internal struggle between its power and its sense of self can be explored in the context of the rescue team's adventures.
Red is always a popular choice. He's the main character in the original games and has a lot of mystery and strength. People like to write about his further adventures or his relationships with other trainers.
The gym leaders are also quite popular. For instance, Brock and Misty. In fanfics, they can be portrayed in different ways. Maybe Brock has a secret mission related to the Pokemon in his gym, or Misty has a special connection to a certain type of Water - type Pokemon that is explored in the fanfic story.
The main character from the game, of course, is a popular choice. Also, Cyrus from Team Galactic is often included as he is such a complex and interesting villain. And Dawn or Lucas, depending on the version, are great to have as they are the typical Pokemon trainers that players can relate to.
Pikachu is always a popular choice. It's so well - known and cute, and imagining it in a transformation scenario, like turning into a human or another Pokemon, can be really interesting. Another popular character would be Charizard. Its cool and powerful look makes for great transformation stories. And of course, Mewtwo. Given its mysterious origin and powerful psychic abilities, a TG story involving Mewtwo could be very engaging.
In Pokemon, Pikachu is always a popular choice. It's the mascot of the franchise and super recognizable. In Digimon, Agumon is really popular. So including them in a fanfic would draw a lot of readers. Also, characters like Ash from Pokemon and Tai from Digimon are well - loved and would be great to have in a crossover fanfic as they could lead the combined adventures.
Ash is always a popular choice. His long - standing relationship with his Pokemon makes him a great character for this kind of fanfic. Also, Pikachu, as it's one of the most well - known Pokemon.
Definitely Ash and his Pikachu. They are iconic in the Pokemon world and having them in a 'pokemon xy' fanfic can add a sense of familiarity and excitement. Serena is also a popular choice as she has her own charm and goals in the 'xy' series. Another character could be Clemont, with his inventions adding an interesting technological aspect to the story.
One idea could be about a new Pokemon joining the rescue team who has a unique power that helps them solve a mystery in the Pokemon world. For example, a psychic - type Pokemon that can read the memories of lost Pokemon to find out where they belong.
There could be a Pokemon that has a really cute appearance but also has extremely powerful abilities. It becomes a fan favorite because it's both adorable and a force to be reckoned with in the story. In the pokemon red sun and blue moon fanfic, this kind of character can attract a lot of attention. It might have some special relationship with the main character, like being their first Pokemon or a Pokemon that saved their life once. This connection makes it even more appealing to the readers.
Ash Ketchum is also very popular. He is the main character in the Pokemon anime series that is based on the Red, Blue, Green and Yellow games. Fanfics often explore different versions of his journey, like what if he made different choices during his Pokemon battles or when choosing which Pokemon to catch. Also, Misty is quite popular. Her feisty personality and her love for water - type Pokemon make her a great character for fanfic writers to explore different storylines with, such as her own solo adventures or her relationships with other characters.