Samus Aran is likely to be one of the main characters. She could be the protector or mentor of the baby in the fanfiction.
There might also be some of the alien species from the Metroid games that interact with the baby. For example, the Chozo, who are an important part of the Metroid lore. They could have a role in the baby's story, perhaps as its creators or guardians in the fanfiction.
Ben Tennyson as a baby is definitely a main character. He's usually the center of the action, whether it's him trying to figure out his powers in a babyish way or just getting into cute little scrapes. Gwen as a baby also often appears. Their relationship as baby cousins can be explored in different ways, like having little spats or being really protective of each other.
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.
Coco herself is, of course, a main character in most coco fanfiction. She is the center of the story, whether it's about her music career or her family relationships.
Willy Wonka is of course a main character. He is often depicted as an eccentric and brilliant chocolatier with many secrets. The Oompa - Loompas also feature prominently as his helpers in the factory, with their unique songs and personalities.
Obviously, the Wonderbird itself is a main character. Often, there might be a young wizard or witch who befriends the Wonderbird.
The main characters that usually show up in 2moons fanfiction include Dain. His role in the original is quite significant, and in fanfiction, he can be put into different situations. For instance, he could be involved in a mystery that was not part of the original plot. Also, Ronark is often seen, especially when it comes to stories about the leadership and power dynamics within the 2moons world.
Typically, the main characters from the original work on which the 'in a heartbeat' is based are often the main focus in the fanfiction. However, sometimes new characters can be introduced to add more depth or drama to the story.
Harry Potter is often a main character. Since he has a connection to the place through Sirius Black, he's frequently involved in the stories set in 12 Grimmauld Place. His experiences and growth as a wizard can be explored in the context of this spooky old house.
Obviously, the King of Enchanters himself is a main character. He is often the central focus of the fanfiction, with his powers and actions driving the story. Then there are his loyal followers or apprentices. They play important roles in assisting him or sometimes causing trouble for him. There may also be rival enchanters who are constantly trying to undermine the King of Enchanters' authority or steal his secrets.
There could be a group of characters led by a wise elder. The elder has some knowledge about the dark below but not enough, so they all embark on a journey together. Each character in the group might have different skills like combat, magic, or navigation that are useful in the exploration.