Well, Mito Uzumaki could be a very popular supporting character. She was Hashirama's wife. In a fem Tobirama fanfic, she could be a friend or an ally to the fem Tobirama. Danzo Shimura might also be included. His shady nature and his different views on the village can create conflict and tension. And don't forget about Jiraiya. His flamboyant personality and his skills as a shinobi can add some humor and action to the story when paired with a fem Tobirama.
One common theme is revenge. If something bad happened to her family or clan, she might be on a path of revenge. There's also the theme of leadership. As a strong female character, she could be taking on leadership roles in her clan. Additionally, friendship can be a theme. She might form deep friendships with other female shinobi or even male shinobi who respect her, which is a contrast to the often - male - only friendships shown in the original context.
Annabeth is often a very popular supporting character. Her relationship with Fem Percy takes on new dimensions. They might have a more competitive relationship as two strong female characters, or a deeper bond based on their shared experiences. Another popular character is Grover. His loyalty to Percy remains the same, but now he may have to protect her in different ways, given her new identity. And of course, the gods themselves play a big role. Poseidon, as Percy's father, has to deal with his daughter being a female, which can lead to some interesting family - dynamic stories.
Well, in many Xenomorph King fanfics, the Ripley - type characters are quite popular. These are strong, resourceful female characters who are not afraid to take on the Xenomorph King. They can be the ones leading the resistance or trying to find a way to co - exist with it. Then there are the alien - hybrid characters. These are characters that have some Xenomorph DNA in them, and they can have a complex relationship with the Xenomorph King, either being drawn to it or fighting against it. Also, the ship's crew members are often used as supporting characters. They can add a sense of chaos and panic as the Xenomorph King attacks their ship.
Brock is often a popular supporting character. He's known for his knowledge about Pokémon, and in Misty x May fanfics, he can offer advice to both of them. Another one could be Ash. Since he's a central figure in the Pokémon universe and has connections with both Misty and May, he can play an important role in the story, like helping them resolve a conflict. Dawn might also be a popular supporting character as she can bring a new dynamic, being another female character with her own set of skills and personality.
There could be a quirky librarian who becomes a key supporting character. They might help Alex with research or give her advice. Another could be the captain of a sports team, if Alex is involved in sports or just has interactions with the team. And don't forget about the school gossip, who spreads rumors and adds to the chaos in Alex's high school life.
Brock is often a popular supporting character. In the original Pokemon Red and Yellow, he's part of the main group, and in fanfics, he can play various roles. He might be the one with all the knowledge about Pokemon types and training tips. Another popular one could be Pikachu. Even though it's the protagonist's main Pokemon, it can have interactions with Misty that are interesting to read. And sometimes, the other Gym Leaders from the games make appearances as supporting characters, like Lt. Surge or Erika.
Sasuke is often a popular supporting character. His relationship with fem Naruto can be complex, especially with the added element of her being half - demon. He might be both intrigued and wary. Sakura could also be popular. She could serve as a contrast to fem Naruto, perhaps initially being more fearful or judgmental of her half - demon nature but then growing to accept her. Kakashi is another one. His wisdom and experience can be crucial in guiding fem Naruto through the difficulties of being half - demon in the village.
The Dragon King himself is often a very popular character. He is usually depicted as powerful, wise, and having a mysterious allure.
Obviously, the female version of Tobirama would be a main character. The baby would also be a central figure. And perhaps some of Tobirama's old comrades like Hashirama could be important characters too. Hashirama might offer his support and wisdom to the female Tobirama during her time with the baby.
Fem Harry herself is often a very popular character in these fanfictions. Her new take on the traditional Harry Potter role brings a fresh perspective. Hermione also remains popular as she is often Fem Harry's closest friend and ally. Viktor Krum is also a popular character as his relationship with Fem Harry can be explored in new and interesting ways in the context of the Goblet of Fire story.