Kuroko as a baby is the central character. But other characters from the opposing teams could also be important in an unexpected way. Maybe a rival team's player discovers Kuroko's situation and has a change of heart, deciding to help rather than take advantage. And of course, Kuroko's schoolmates who are not on the basketball team might also get involved, adding a different perspective to the story as they are not as familiar with Kuroko's basketball skills but still care about him as a person.
Ginny herself would obviously be a main character, even as a baby. Then Harry, Ron, and Hermione are likely to be main characters as well. They are her friends and would be the ones most likely to try and solve the problem of her being turned into a baby. They would be involved in taking care of her and finding a way to reverse the transformation.
Obviously, Keith himself in his baby form would be a main character. His friends like Lance, Pidge, Hunk, and Shiro would also be main characters as they are the ones who would most likely be taking care of him or trying to find a cure. Maybe Allura and Coran could also be important characters, helping out with their knowledge of Altean technology or magic that might be related to Keith's transformation.
Well, the plot could involve his teammates having to take care of him. They might struggle at first because Kuroko as a baby can't communicate in the same way. Maybe they have to figure out his needs like when he's hungry or sleepy. And they could also try to keep it a secret from the other teams to avoid any distractions during games.
His friends from his original life could be main characters. For example, if he has a best friend, that person would play an important role in helping Kuroko adjust to being a girl. Maybe they would offer support and advice, or just be there to listen when Kuroko has a tough time dealing with the changes.
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, Ash would be a main character even in his baby form. Pikachu is bound to be a key character as well, being Ash's closest Pokémon partner. His other Pokémon like Bulbasaur, Charmander, and Squirtle could also play important roles in protecting and taking care of baby Ash.
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.
Obviously, Shadow himself would be a main character even in his baby form. Sonic would also be a key character as he is often the hero in these Sonic - related stories. He would likely be involved in trying to figure out how to turn Shadow back. Tails could be another main character as his intelligence would be useful in finding a solution to Shadow's transformation. He might study the cause of it and try to come up with a remedy.
Obviously Naruto himself is the main character as he is the one who turns into a baby fox. His friends like Sasuke and Sakura would also be involved. Sasuke might be initially confused but then determined to help Naruto. Sakura would probably be more nurturing and try to take care of the baby fox Naruto. Kakashi Sensei could also play an important role, using his wisdom to figure out what happened to Naruto and how to reverse it.
Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.