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.
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.
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.
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.
There could be his friends like Ochaco Uraraka. She might be in a position where she's trying to understand Izuku's situation and help him. Maybe she tries to convince the people forcing Izuku to give him some space.
Obviously, Izuku is one of the main characters since the title mentions him. He's the demigod in this fanfiction.
There could be a mysterious Pokemon that Izuku discovers early on in the story. This Pokemon could have some sort of special power or connection to Izuku. It could be a key factor in his development as a Pokemon fan and as a hero. The Pokemon might not be fully understood at first, and Izuku's journey to figure out its secrets could be a major part of the story.
Perhaps a jealous classmate who has always been in Izuku's shadow. Now that Izuku is a girl, this classmate sees an opportunity to undermine her. They spread rumors about how Izuku's transformation is some sort of cheat or that she's not the same reliable person she was as a boy. This classmate's actions and attempts to sabotage Izuku would make them the main antagonist.
There could be other U.A. teachers like All Might. He might play a role in Izuku's development even though Midnight has adopted him. Another character could be Bakugo. His relationship with Izuku could change or be explored further in this fanfiction.
His classmates like Bakugo could be a main character. Bakugo might be one of the first to notice the strange changes in Izuku and could be both scared and angry about it. He might try to confront Izuku, which could lead to some intense scenes.
The main characters are clearly Izuku and Monoma. Izuku, also known as Deku, is the protagonist of the main 'My Hero Academia' series, a kind - hearted but often underestimated boy with a powerful quirk. Monoma, on the other hand, is from a different class in U.A. High School. He has a quirk that allows him to copy others' quirks for a short time. Their interactions, especially when Izuku snaps at Monoma, are the central focus of this fanfiction.