The person who stabbed her could also be a main character. They might have their own motives, like being a jealous ex - lover or a hired assassin. Their backstory could be explored as the story progresses.
There could also be a friend or family member of the girl who becomes a main character. For example, her best friend who is by her side throughout her recovery process, or her father who is determined to find the person responsible for stabbing his daughter and make them pay.
One possible plot development could be the immediate rush to get medical help. Maybe some bystanders call an ambulance and the girl is rushed to the hospital where the doctors fight to save her life.
Morgana could be a likely antagonist. She has been at odds with Merlin in the series and has her own motives and powers. She might see stabbing Merlin as a way to further her plans against Camelot or to eliminate a threat to her own power.
The girl and a group of small, intelligent dinosaurs like the velociraptors could be the main characters. The velociraptors might initially be curious about the girl and then they start to help her in her journey through their world. There could also be an old, wise dinosaur that gives the girl advice on how to survive in their world.
It could be two young people, like a college student and a barista. The college student often stops by the coffee shop where the barista works and they start to form a connection over time.
Obviously, the girl in the tower is one of the main characters. There could also be a knight in shining armor who comes to rescue her. Maybe a wizard or a witch who has some connection to the tower, either as its keeper or someone who wants to use the girl's power for their own gain.
Maybe there's a mysterious visitor who comes to the ward from time to time. This person seems to know something about the girl's past or has some connection to her that she doesn't remember at first. As the story unfolds, this visitor becomes a key figure in the girl's life in the ward.
The main characters could be a nerdy 'cherry boy' who is always buried in his books and a sporty girl. The boy might be really smart in academics but lacks social skills. The girl, on the other hand, is confident on the sports field but not so good in studies. They could help each other out, with the boy tutoring the girl and the girl teaching the boy how to be more confident in social situations.
I think Riley and Maya are the obvious choices for main characters. Riley with her wide - eyed wonder and Maya with her edgy personality. But Farkle Minkus could also be a main character in this fanfiction. His intelligence could be put to use in different situations in Texas, like helping to solve a local mystery or coming up with innovative ideas for a community project. Also, Smackle could join them, adding her own brand of quirkiness to the group.
The main character could be the girl who owns the stained socks. She could be a curious and adventurous girl who loves her unique socks. Another character could be her best friend who always teases her about the stained socks but secretly is quite intrigued by them.
Obviously, the little girl is one of the main characters. And a friendly dinosaur, like a small Stegosaurus, could be another main character. They are the focus of the story as they interact with each other.