Maybe it's a story about a mentor and a protege. The mentor has found an amazing opportunity for the protege and is urging them with 'baby just say yes'. So the main characters would be an experienced person and a young, inexperienced one.
Perhaps the story is set in a magical or fictional world. There is a task or a quest that needs to be done, and a guide or a partner is asking 'baby just say yes' to embark on this adventure. The story then unfolds with all the challenges and revelations during this journey.
It might imply that in a love story, having the courage to say yes leads to something wonderful.
It depends on the specific 'yes day story'. It could be a family with parents and children as the main characters. For instance, in a common family - based 'yes day story', the mom, dad, and their two kids are the central figures. They are the ones experiencing all the yes - day adventures.
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.
I'm not sure exactly who the main characters are without more context. But it could be a family, perhaps a mom, a dad, and their kids. They would be the ones experiencing the 'yes day' and all the crazy things that come with it.
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.
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.
Well, perhaps the main characters are a male and a female. They might be coworkers or neighbors who initially don't think much of each other but then gradually develop a friendship. They could have unique personalities. For example, one could be very outgoing and the other more introverted. Their differences might be what makes their friendship interesting and the focus of the short story.
A writer who wants to make it clear that their work is not about war. They might be writing a story about a different topic, like a family drama or a story about a journey, and don't want the readers to assume it's a war story.