There could be characters who have unique abilities or characteristics within the context of the gender game. For example, a character who has the power to manipulate the rules of the gender game in some way might be very popular. Also, characters who form strong relationships, whether it's friendship or romance, are likely to be liked by readers. These relationships add depth to the story and make the characters more relatable.
I'm not entirely sure which specific characters are popular in it as it can vary from reader to reader. However, typically, the main protagonists who are directly involved in the gender game and are trying to overcome challenges related to it tend to be popular. They are often the ones driving the story forward and facing the most difficult situations.
Kakashi is a character that some fanfictions play with in terms of hidden gender. As a very popular and mysterious character, changing his gender can bring out new aspects of his personality. His relationship with his students, especially Naruto, Sasuke and Sakura, would be very different. Also, Itachi could be a character where a hidden gender aspect in fanfiction can create a whole new set of storylines and emotional arcs.
Natsu is quite popular. Since he's one of the main characters, seeing him as a female brings a whole new dimension to his relationships and his fiery personality. People like to imagine how his interactions with Happy and the rest of the guild would be different.
Probably Female Thor. She brings a new and exciting take on the character. Her story can be different from the male Thor's, like how she might handle her relationships with other Asgardians or mortals. Another popular one could be Male Black Widow. People are often interested in seeing how a male version of this complex and deadly assassin would operate. And also Female Iron Man. The idea of a woman in the role of Tony Stark, with her own unique style and approach to technology and heroism, is really appealing.
Definitely Naruto himself (as the gender - bended version). Since the whole concept often revolves around him changing genders, he's at the center of most of these fanfictions. Another popular character would be Sasuke. His relationship with a gender - bended Naruto always makes for an interesting dynamic.
Definitely Luke Skywalker and Leia. They are iconic characters, and gender - swapping them allows for a lot of creative exploration. But also, characters like Qui - Gon Jinn are getting more popular to be gender - swapped. His role as a mentor could be very different if he were a female. And Palpatine, too. A female Palpatine could be a very different kind of villain, with different motives and methods of achieving power in the Star Wars galaxy.
Definitely Marinette and Adrien. Since they are the main characters, when they are gender - swapped, it creates a whole new dynamic. People are really interested in seeing how a male Marinette would handle the Ladybug powers and responsibilities, and the same goes for a female Adrien as Chat Noir.
Rukia is also a popular character in ichigo game fanfiction. Her relationship with Ichigo, whether it's a deepening friendship or a potential romantic interest, is often explored in these stories. She has her own unique powers and personality that make her an interesting addition to any fan - created plot.
Madeline is definitely one of the most popular. Since she is the main character in the game, many fanfictions revolve around her, her journey, and her growth.
Obviously, the gender - bent Batman is a popular character in these fanfics. Writers often like to explore how this new version of the Dark Knight would interact with the existing Gotham City characters.
Harry himself is also a common character for gender - bending. As the main character, changing his gender can completely shift the narrative. It allows for new interpretations of his hero's journey and how he deals with the challenges of the wizarding world as a female. Fans are interested in seeing how a female Harry would handle things like the Triwizard Tournament or facing Voldemort.