Perhaps a mysterious stranger. This stranger could have some connection to the reason why Edward turned into a baby. And then there's Edward's pet dog. The dog might be confused at first but then becomes very protective of baby Edward, adding an interesting dynamic to the story.
There could be a prison guard who takes an interest in Edward's case. This guard might be sympathetic and start to help Edward in small ways, like getting him access to a lawyer or better living conditions in jail.
A young girl who is also homeless might be another main character. She could teach Edward the ropes of surviving on the streets, like where to find the best places to get food donations or how to stay safe at night. And they could develop a special bond during their time together.
There could be a girl named Bella who might be his love interest. She could be the one trying to help Edward regain his memory. Another character could be his best friend, Jacob, who is always by his side, protecting him while he's in this vulnerable state.
One of the main characters might be Alice. Her ability to see the future could be crucial in helping Edward navigate the political and dangerous situations in his role as king. Also, Jasper could be there. His experience in combat and control of emotions could be useful for Edward's rule. And then there's Rosalie, who might have her own opinions on how Edward should run the kingdom and could be a source of conflict or support depending on the story.
Edward, of course, would be one of the main characters. And there could be a female character, like Isabella, who might be Edward's love interest and the potential mother of his baby. Maybe there's also Edward's best friend, Jacob, who gives him advice on this journey of wanting a baby.
Well, it could be that Edward is cursed by a powerful witch. The witch might be angry with him for some reason. And so, he turns into a baby. His friends then have to figure out how to break the curse while taking care of baby Edward. They might go on a journey to find a special herb or a magic item.
His family members are quite popular. Maybe a sister or a brother who has a different view on the family wealth. They can bring in family drama. Also, the love interest is very important. It could be someone he meets by chance, like a struggling artist or a young journalist who is intrigued by him and his world.
Alice is often a great character in such fanfiction. Her ability to see the future can add a lot of twists and turns to the story. For example, she might foresee a danger to Edward and Bella's relationship and try to prevent it.
Alphonse Elric is often present. He might play a role in helping Edward understand and use his new gamer - like abilities in relation to alchemy. For example, he could be the one who discovers a new aspect of Edward's powers.
Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.