Professor Oak can be included. He could be the one who gives Red and Blue important missions or tasks. His wisdom and knowledge of Pokemon can guide the two protagonists in their journey. For example, he might provide them with crucial information about a rare Pokemon they need to find to achieve their goals in the fanfiction.
Pikachu is an obvious choice. Since Pikachu is Red's iconic Pokemon, it can play a significant role in the fanfiction. It could be used to show the bond between Red and Blue in different ways. Maybe Pikachu gets lost and both Red and Blue have to work together to find it, which in turn strengthens their relationship and trust in each other.
Obviously, Red and Blue themselves are must - includes. Their rivalry and different personalities make for great storytelling. Green could also be included as she adds an interesting dynamic, especially with her own goals and relationships with the boys.
One idea could be a story where Red and Blue have to team up to face a new, extremely powerful and mysterious Pokemon that threatens the entire region. They start off with their usual rivalry but gradually learn to trust and rely on each other more as the adventure progresses.
Catwoman can be a great character in such fanfiction. She has her own moral code that often clashes with Batman's. Her presence can create more tension and also add an element of romance. She could also interact with Red Hood in interesting ways, perhaps seeing some of herself in him.
One popular character is often the player - character from Pokemon Red or Blue, who is now exploring Alola. Their experience and knowledge from Kanto make them stand out in the new region.
The protagonist from the original Red and Blue games often makes an appearance in these fanfictions and is quite popular. People love seeing how they would interact with the Alola region and its Pokemon.
A girl character who is a Pokemon healer is also quite popular. She has a special ability to heal injured Pokemon quickly and efficiently. In the fanfiction, she travels around the Red and Blue regions, helping trainers and their Pokemon, and sometimes gets involved in battles when she has to protect the Pokemon she's healing.
Well, for Red vs Blue, Tex is quite popular in fanfiction. She's strong, mysterious, and her relationship with Church is always interesting to explore. In RWBY, Weiss is a favorite among fanfic writers. Her growth from a somewhat cold character to a more empathetic one is a great arc. When it comes to the crossover fanfiction, Caboose is often used. His naivete can lead to some really funny and heartwarming moments when interacting with RWBY characters.
Often, Naruto and Sasuke can be main characters in Naruto red and blue fanfiction. Naruto could be associated with the blue side, perhaps representing his calm and friendly nature that lies beneath his energetic exterior. Sasuke, on the other hand, might be linked to the red side, symbolizing his passionate and sometimes vengeful motives.
Red and Blue themselves are very popular. Red, being the silent but strong protagonist of Pokemon Red, and Blue, his rival, often star in fanfictions. Their relationship, which is a mix of rivalry and friendship, is explored in many stories.