Aside from Sakura in her baby state, there could be a mysterious character who caused the transformation in the first place. This character could be either a villain who did it on purpose or an accidental culprit. Then, there would be the group of friends or allies that Sakura has, like the other card captors. They would all work together to deal with the situation, with each bringing their own skills and knowledge to the table.
Sakura herself would obviously be a main character, just in her baby form. Her best friend could also be a main character. For example, if she has a friend like Tomoyo, Tomoyo would likely be very involved. She would be trying to figure out how to help Sakura and also taking care of her, making sure she has cute clothes even as a baby.
Tomoyo would also be an important character. She would be very worried about Sakura. She might use her skills in making costumes to create some special baby clothes for Sakura. And she would be constantly by Sakura's side, taking care of her and trying to comfort her.
Obviously, Sakura would be one of the main characters. There could also be her best friend, who tries to comfort her while she is crying. Maybe a mysterious stranger who witnesses her crying under the sakura trees and is intrigued by her sadness.
The main character is surely Sakura as it is in the title. The baby is another main character as the whole concept of the fanfic seems to revolve around Sakura hiding this baby. There could be antagonists as well, perhaps some people or forces that are looking for the baby, but we don't know who they are specifically just from the title 'sakura hiding baby fanfic'.
Well, Sakura Haruno is obviously one of the main characters. The baby, of course, is also a key character. Depending on the plot, other characters like Naruto or Sasuke might also be involved. For example, Naruto could be the one who helps Sakura find the baby in the first place.
Well, obviously baby All Might is a main character. He's likely to be the center of the story, with his cute and innocent personality.
The main character is Sakura. Her friends, say Tomoyo who might be there to offer fashion advice as Sakura's body changes during puberty. Then there could be a boy she has a crush on, which is a common theme in puberty - related stories. And her parents, whose reactions to Sakura's growing up, whether it's strict or lenient, can play a big role in the story.
In a 'sakura turned into a baby fanfic', it could start with Sakura getting too close to a mysterious portal. The portal's energy zaps her and she is transformed into a baby. Her closest companion might notice some strange behavior before the full transformation. Once she's a baby, they need to protect her from those who might take advantage of her vulnerable state. They also have to deal with the challenges of taking care of a baby, like feeding, changing diapers, all while desperately looking for a solution to return her to normal.
Sakura's friends like Naruto and Sasuke could be main characters. They might be the ones trying to find a way to save her. They could be using their skills and powers to look for a cure or a solution.
Obviously, Nightwing himself, even as a baby, would be a main character. His friends in the Bat - family like Batman, Robin, and Oracle would also be main characters. They would be the ones dealing with the situation of baby Nightwing.
Sakura could be a main character. She might be the one who takes on a motherly role towards baby Naruto. She'd be very protective and try to teach him basic things. Kakashi could also be important. He'd use his wisdom to figure out how to deal with this situation and also keep an eye on Naruto's safety.