Well, obviously Izuku is one of the main characters, even when he's turned into a cat. His friends like Bakugo and Ochaco are also likely to be main characters. They would be the ones trying to figure out what happened to Izuku and how to turn him back. And maybe All Might could also be a main character, if he gets involved in the whole situation to help his young protege.
A more emotional plot could be Izuku as a cat being found by a villain who initially wants to use him against the heroes. But over time, the villain starts to develop a strange bond with the cat - Izuku. This then makes the villain question his own motives and actions.
One idea could be that Batman is turned into a cat by a villain's new experimental device. As a cat, he has to rely on his natural feline abilities like stealth and agility to still protect Gotham. For example, he could use his cat form to get into small spaces in buildings to gather evidence or spy on criminals.
Well, one possible plot could be that his friends have to take care of him. They might struggle at first, not knowing how to handle a baby Izuku. For example, Bakugo could be really frustrated but gradually learn to be gentle. Ochaco might be really good at soothing baby Izuku when he cries.
Definitely his friends from U.A. High. Bakugo would be an interesting character in this scenario. He might initially be annoyed but then develop a soft spot for baby Izuku. Ochaco would also be there, using her quirk in creative ways to take care of him, like floating toys to keep him entertained.
Izuku herself is obviously a very popular character in this type of fanfiction. Since the whole story revolves around her transformation into a girl. Her friends like Ochaco Uraraka also tend to be popular as they often play important roles in Izuku's new life as a girl. Their relationship might change and develop in interesting ways.
Well, in some fanfictions of this kind, Izuku as a girl might discover new aspects of her Quirk. Since the physical change could potentially affect how she uses her power. Also, there could be a plotline where she has to deal with romantic interests in a new way. As a girl, she might attract different kinds of attention from her male classmates, and she has to navigate through those new feelings and situations.
Another aspect is the character development. He has to learn to control this new form, which means dealing with new instincts and emotions. It's like he has a whole new side of himself to explore and understand. For example, in some fanfictions, his dragon form makes him more confident in some ways but also more reckless in others. This internal struggle is really engaging to read about. Moreover, the relationships around him change too. His friends might be in awe or a bit scared at first, and how they all adapt to this new Izuku is a big part of the story.
Well, he might find it really hard to operate his Bat - gadgets as a cat. His paws are not made for pushing buttons or typing on keyboards. So he has to figure out a way to adapt. Maybe he uses his cat claws to scratch out messages for Alfred to understand what he wants to do.
Well, often he has to figure out how to communicate. He might try to use his body language in creative ways to get his friends' attention. For instance, he could lead them to where danger is by pawing at their feet and then running in the direction they need to go.