Obviously, Itachi as a baby would be a main character. Sasuke would also be a key character as Itachi's brother. Their relationship would be very different now that Itachi is a baby. Naruto could also be involved, perhaps being curious about this strange transformation and wanting to help out in some way.
The main characters would be Itachi as a baby, for sure. Then, the people in his village, like the elders or other ninja who knew him before. They would be trying to figure out what to do with this baby Itachi. Additionally, any friends he had made during his time as a normal ninja could be main characters. They might be worried about whether he'll ever turn back to normal and how to keep him safe in the meantime.
Obviously, Deku himself would be a main character even though he's turned into a baby. His friends like Uraraka, Iida, and Bakugo would also be main characters as they would be involved in taking care of him and trying to solve the problem.
Obviously, Naruto himself is the main character in this situation. His friends like Sasuke and Sakura are very likely to be involved. They would be the ones taking care of him, trying to figure out how to turn him back to normal. Kakashi Sensei would also probably be there, guiding them and trying to keep Naruto safe.
England could be one of the main caretakers. Despite their past differences, he would feel a sense of responsibility. Another could be Canada. Since they are related, Canada would surely step in to take care of his baby brother.
The plot could be centered around the chaos that ensues when Itachi turns into a baby. His enemies might see this as an opportunity to attack, thinking he's vulnerable. However, his allies would protect him. And as they take care of him, they might discover new things about Itachi's past or even his true nature that was hidden before. There could be many comical scenes like when they try to diaper him or feed him baby food and he makes a fuss.
Obviously, Ash and Erica would be the main characters. Ash is the one who undergoes the transformation, so he's central to the story. Erica is the one who causes the change, so she also plays a crucial role.
Obviously, Ash would be one of the main characters, since he's the one turning into Victini. Pikachu would also be important as Ash's most loyal Pokémon. And Victini itself could be a key character, either as a part of Ash now or as an entity that helps Ash understand his new form.
Obviously, Harry Potter himself would be the central character as he is the one undergoing the transformation into a born vampire. His friends like Hermione and Ron would also be important characters. They would likely play a role in either helping Harry adjust or being conflicted about his new state.
His siblings would also be key characters. They might play roles such as the protective older brothers or sisters, or the ones who sometimes tease Ron because he's the youngest. Maybe there could be a particularly close - in - age sibling who has a love - hate relationship with Ron due to his baby - of - the - family status.
Obviously, Jaune would be one of the main characters as the title implies he has a baby. The baby itself could also be considered a main character in a sense, as the whole story might revolve around it.
Pain could also be a major character. His ideology and the Akatsuki's goals under his leadership might be affected by the presence of baby Naruto. He might see Naruto as a potential threat to his plans or as an opportunity to change the Akatsuki's path. Also, Konan might be involved as she often has a more empathetic side compared to some of the other Akatsuki members and could play a role in taking care of the baby.