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.
There could also be antagonists in the story. If the baby has some special quality, there might be people who oppose Jaune and the baby. These antagonists would be important characters as they would drive the conflict in the story.
Obviously, Jaune would be one of the main characters. And considering the RWBY series, Ruby, Weiss, Blake, and Yang are likely to be involved as well. They are the main protagonists of RWBY and often interact with Jaune. So it's possible that one of them could be the recipient of Jaune's nightly visit.
Ruby would most likely be involved. She's the leader of Team RWBY and has a strong sense of justice. She would be determined to find Jaune no matter what. Weiss could also be involved as she has the resources and skills to help in the search. Her family's influence and her own combat abilities would be useful.
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.
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, 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.
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.
Most likely, the main characters would be the Loud siblings. They are at the center of the 'Loud House' series, so in a fanfiction about changing the baby, they would probably play important roles.
Obviously, Severus Snape and baby Harry Potter are the main characters. Severus would be in the role of the caregiver, which is a huge departure from his usual role in the original series.
Obviously, the Cullens would be the main characters. Carlisle, as the patriarch, would play a crucial role in raising Bella. Esme would be like a mother figure to her. Edward would also be important, his relationship with Bella evolving from something different than in the original story. And of course, Bella herself is a central character.
Even some of the antagonists could potentially help Jaune if there is a complex relationship developed in the fanfiction. For example, if an enemy realizes that Jaune's death or continued injury would disrupt some sort of balance, they might step in to heal him, albeit reluctantly.