There could be the narrator, who is the one trying to get away from Sakura. Maybe there's also a best friend of the narrator who encourages them to break free from Sakura. For instance, this friend might see the negative impact Sakura has on the narrator's life and gives advice on how to get away.
There could be the prison guards. Some might be cruel and strict, while others could be more sympathetic. Also, other inmates. Maybe an older, experienced inmate who becomes a mentor - like figure to Sakura.
Well, it could be about a character named Sakura who has some sort of negative relationship with the narrator. Maybe Sakura has done something wrong or annoying to the narrator, and the story is about the narrator's attempts to distance themselves from her. For example, Sakura might be a very clingy friend, and the narrator wants to break free and have their own space.
There could be Naruto. Since he's Sakura's friend, he would likely be involved, trying to help her deal with the curse mark. He might use his own powers and experiences to support her.
It could be her friends from the present who somehow also get sent to the past with her. Maybe Tomoyo, who could be helpful in creating new outfits for Sakura to blend in with the past's fashion.
There could be the patients, of course. Each patient might have their own unique backstory and personality. Maybe there's a young patient who is dealing with trauma, and Sakura has to find a way to reach out to them.
The Akatsuki members would be the main ones. It could include Itachi, who is known for his complex character and powerful abilities. His relationship with Sakura could be an interesting aspect, whether it's one of mutual respect or a more adversarial one.
Sasuke is also frequently present. His complex relationship with Sakura can be further explored in the fanfiction. The reincarnated character could either get involved in their relationship or try to change the dynamics between them.
There could be original characters created specifically for this fanfic. These new characters might be the source of Sakura's anger. For instance, a new villain who has targeted Sakura or her loved ones. Or it could be a rival who constantly undermines Sakura, causing her to be angry. Additionally, characters from other related series might make cameo appearances if it's a crossover - type fanfic.
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.
One possible reason could be that Sakura has grown tired of Sasuke's cold and distant attitude. In many Naruto storylines, Sasuke is often preoccupied with his own goals and doesn't show much warmth towards Sakura. So, in the fanfic, she might have finally reached her limit and told him to get away.