Sakura is the main focus. Alongside her, there could be a new character who she meets by chance and who somehow gets involved in her secret. This new character could be from a different place or group and has their own motives regarding Sakura's secret.
Sakura's friends like Naruto and Sasuke could be main characters. They might be the ones trying to find a way to save her. They could be using their skills and powers to look for a cure or a solution.
It could be her best friend. Her best friend might be the one who accidentally discovers Bella's secret in the fanfiction. And then they have to deal with the situation together, whether it's keeping the secret or helping Bella face whatever the secret entails.
Lucy herself would surely be one of the main characters. And maybe she has a best friend in the story, like a character named Lily who helps her through all the challenges.
Sakura herself is obviously a main character. There could also be Naruto and Sasuke. Maybe Sakura misses her chance with Sasuke because she didn't speak up when she had the opportunity. Naruto might be involved as a friend who tries to encourage her but fails. Another character could be Kakashi, her sensei, who might have given her advice that she didn't follow, leading to her missing the chance.
Since it has 'Sakura' in the title, Sakura is likely to be one of the main characters. There could also be other characters who are related to the reason for the apology or those who are affected by it. Maybe there's a character who is on the receiving end of the apology.
Jace would likely be an important character. Given his relationship with Clary, he would probably be deeply affected by the news of the baby. His reaction could range from shock to protectiveness.
One character could be Naruto. He might be Sakura's friend, but completely oblivious to her hidden pain. He's so focused on his own goals that he doesn't notice Sakura's struggle, which could add to her feelings of loneliness.
Obviously, Sakura would be one of the main characters. There could also be her best friend, who tries to comfort her while she is crying. Maybe a mysterious stranger who witnesses her crying under the sakura trees and is intrigued by her sadness.
Maybe there's a mentor figure in Sakura's life. After Sakura makes a mistake, the mentor could guide her on how to make a proper apology and help her understand the importance of taking responsibility for her actions. This character could add depth to the story by providing wisdom and advice.
The main character is the sakura secret princess. Alongside her, there could be a young and adventurous boy from a neighboring village who stumbles upon her secret. There might be a powerful queen mother in the story who has her own motives regarding the princess's secret. And then there could be a mysterious figure in the shadows, an agent of an unknown force, who is constantly watching the princess and trying to figure out her secret, adding an element of mystery and suspense to the fanfic.