He might do it for a bet. Just like some friends might bet on who can pull off a crazy stunt, his friends could have bet him that he can't dress like a girl and act the part for a certain period.
Well, it depends on how the author of the fanfic has characterized Figure 13. However, if the premise is that Figure 13 is a girl in this fanfic, then that's how it is within the context of that particular story. Different fanfics can have different interpretations and characterizations, but in this one, it's stated that Figure 13 of Izuku is a girl.
Obviously, Bakugo would be one of the most affected. His relationship with Izuku has always been complex, and Izuku becoming a girl would surely change the dynamic between them. He might have to re - evaluate his feelings of rivalry and superiority. All Might would also be affected. As Izuku's mentor, he would need to adjust his training methods and support system to fit Izuku's new situation as a girl.
Izuku might become more self - conscious about her appearance in this fanfiction. Since she is a girl, she could face different beauty standards, which may influence her confidence in different ways compared to when she was male - presenting in the original story.
Self - discovery is a common theme. Izuku has to figure out who she really is as a trans girl in a society that has certain expectations. Another theme is acceptance, both self - acceptance and acceptance from others like friends, family and the community.
Well, it could start with Izuku waking up as a girl one morning with no explanation. He (now she) tries to keep it a secret at first but then realizes it's not possible. The story could focus on how Izuku's relationships change. For example, Bakugo might initially tease her more harshly but then come to respect her in a different way. And Midoriya could find that she has a different approach to using her quirk as a girl, maybe being more strategic or having different physical capabilities that she has to learn to master.
A great 'izuku is a girl' fanfic could focus on her relationship with other characters. Maybe she has a closer bond with Ochaco as they are both girls now. They could support each other more in their hero training, and Izuku could also inspire Ochaco in different ways. It could be really interesting to see how the dynamic between the characters changes with this gender swap.
Perhaps in this fanfiction, Izuku as a girl might face different challenges in the hero world. Society could have different expectations of a female hero, which would add a new layer to her character development. She might have to deal with sexism while trying to prove herself as a capable hero, just like real - world female heroes often do.
In the 'izuku becomes a girl fanfic', perhaps the plot involves Izuku adjusting to her new physical form. She might struggle with things like different clothing styles and how society views girls in the hero world. Also, her relationship with other characters could change. Her friendship with Ochaco might become more complex as they now have more in common as girls. And Bakugo might react in a really interesting way, either being more protective or even more competitive towards her.
One common theme is the discovery of new powers. Since Izuku is often associated with his journey to gain powers in his original story, when paired with Power Girl, there could be a focus on how he either adapts to her powers or discovers new ones through their connection. Another theme could be the culture shock. Power Girl comes from a different world in a sense, so Izuku dealing with that aspect is often explored.