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.
While Mikey and Donnie are the main focus, sometimes their older brothers Leo and Raph can also play important roles in the fanfiction. For example, they might be searching for the missing baby turtles, or they could be the ones who teach Mikey and Donnie important lessons when they finally reunite.
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, 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.
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.
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.
April and Casey might also be part of this fanfiction. April could use her reporting skills to try and figure out what's going on with Mikey's mutation, and Casey would be ready to fight any new threats that come along because of Mikey's changed state. Also, Splinter, as their sensei, would be guiding them all through this difficult time, providing wisdom and training to help Mikey control his new powers if he has any.
Splinter might be the main helper. He has raised the Turtles, so he has experience with taking care of young ones. He could teach the other Turtles the proper way to hold baby Mikey, how to soothe him when he cries, and give them advice on how to keep him clean and healthy. Splinter's wisdom would be really important in this situation.
Friendship is also a big theme. His relationships with other characters, how they form and develop, are often explored in 'baby boy mikey way fanfic'. For example, he might make a new friend who changes his perspective on things.
One popular theme could be baby Mikey's adventures in a magical world. For example, he might discover a hidden land full of talking animals and have to solve problems to find his way back home.