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.
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.
Well, obviously Blaine is one of the main characters. And the baby is also a central figure. There might also be other characters like Blaine's friends or family members who play important roles, such as his partner who helps him take care of the baby or his parents who offer advice on raising the child.
Deadpool is the central figure for sure. The baby is another main character as the story revolves around their relationship. There may be other characters like Vanessa if the fanfiction decides to bring her into the story. She could be either a co - parent or have some influence on how Deadpool raises the baby.
Perhaps there are antagonists in the form of other players. These could be characters who are overly competitive or have some sort of grudge against Jaune. Their actions and decisions during the games would drive the conflict and keep the story moving forward.
Another plot could involve the baby having some special abilities or a unique origin. This could lead to Jaune having to protect the baby from those who might want to use it for their own purposes. For example, there could be villains who are after the baby because of its power.