The baby metroid itself can be considered a central 'character' in its own right. It has its own behaviors and reactions that can influence the other characters around it. For example, its need for energy or its growth spurts might force the other characters to make certain decisions. And of course, there may be characters from the Metroid universe's background lore that show up to interact with the baby metroid situation, like the Chozo who have a connection to the Metroids in the official lore.
Samus Aran is likely to be one of the main characters. She could be the protector or mentor of the baby in the fanfiction.
One common theme could be the growth and development of the baby character in the Metroid universe. Maybe it's about how the baby adapts to the dangerous and strange environment.
One interesting concept could be the growth and development of the baby metroid. Maybe it discovers new abilities as it matures in the fanfiction.
Obviously, baby Rosalina herself is the main character. There might also be her mother or caretaker in some fanfictions. And sometimes, there could be other baby versions of well - known space - related characters like baby Mario or baby Luigi who interact with her.
Obviously, Mikey himself as a baby is a central character. His family members often play important roles too. For instance, his older brothers or sisters who might protect him or play with him.
Obviously, baby Quasimodo himself is a central character. Then there's Frollo, who plays a significant role in Quasimodo's life in the original story and would also be important in fanfiction about his baby days.
In 'bella as a baby fanfiction', apart from Bella and her immediate family, there could be characters who have a special connection to Bella's future self. Maybe there's an old wise woman in the neighborhood who somehow senses that this baby will grow up to be someone special. So she starts to protect Bella from an early age, becoming an important character in the story. And also, if it's set in a particular fictional universe, there could be some creatures or beings that are drawn to Bella because of her unique presence even as a baby.
Well, obviously baby Mew itself is a common character. There are often other Pokémon characters like Pikachu. Pikachu might be a friend or a protector to baby Mew in the fanfiction. Another common character could be a Pokémon trainer. The trainer might find baby Mew and take it under their wing.
Survival in a new environment is another theme. Naruto characters suddenly in the Metroid universe, or vice versa, have to figure out how to survive. They need to adapt to new threats, whether it's the space pirates in Metroid or the powerful shinobi in Naruto. This theme adds a lot of excitement and adventure to the fanfictions.
Another common character could be Supergirl. She might be like a cool older sister figure to baby Conner Kent. She could help him understand his powers better, especially since they both share the Kryptonian origin. She might take him on little flying lessons or tell him stories about Krypton. And of course, there could be other alien characters from Krypton who show up to guide or protect baby Conner as well.