Another potential main character could be Kakashi. As Sakura's teacher, he might feel responsible for her well - being and try to understand what has caused this change in her. He could be the one guiding her back to a more positive state of mind or at least trying to figure out what's going on with her.
Sasuke could also be a main character. Given their complex relationship history, Sasuke might be involved in some way. Maybe his actions or presence in the story are part of the reason why Sakura has reached this state of not caring anymore.
It could be Naruto. Since he has always been a big part of Sakura's life, he might be one of the main characters trying to figure out what's wrong with Sakura and how to bring back the old her.
Tsunade could be a significant character. As the Hokage and a medical expert like Sakura, she could have some knowledge about the crystal. She might offer advice or warnings to Sakura. Then there's Shizune, who could assist Tsunade in dealing with Sakura and the crystal situation. And of course, Orochimaru might be a potential villain, interested in the crystal's power for his own dark experiments.
Perhaps in the 'sakura doesn't care anymore fanfiction', Sakura has been betrayed one too many times. She could have been let down by her friends or mentors. As a result, she withdraws from the world around her and stops caring about the things that used to matter to her, like the outcome of battles or the well - being of the village. This change in her attitude would have a significant impact on the story's dynamics.
Another aspect is how other characters react to her change. Some might be confused, some might try to bring back the old Sakura, and others might be intrigued by this new version of her. This can create a lot of interesting interactions and dialogues in the fanfiction.
It could also be developed by introducing a new character or a new situation that makes her question the things she used to care about. Maybe she meets someone who has a completely different outlook on life and it makes her reevaluate her own values, leading to her not caring anymore. In the fanfiction, this can be explored in depth through her internal monologues and conversations with this new character.
Well, Naruto would be a likely character. He and Sakura are friends, and he'd probably be really worried about her. He might visit often and try to cheer her up with his goofy antics. Another could be Sasuke. Even if he's a bit more aloof, his presence would add an interesting dynamic. He could come to see her out of his own sense of obligation or perhaps a deeper, unspoken feeling.
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.
There could be some original characters created specifically for this fanfiction. Maybe a mysterious mentor figure who claims to be able to help Sakura control her dark side, but has their own ulterior motives. Also, her sensei Kakashi could be there, trying to figure out how to guide Sakura through this difficult time.
Naruto could be a great addition. His friendship with Sakura and his complex relationship with Kabuto in the original series could add an interesting dynamic. For example, Naruto might suspect something is going on between Sakura and Kabuto and try to interfere or understand what's happening.
A young girl who has always been fascinated by Sakura could be a main character. She might be on a journey to see the most amazing Sakura blossoms.
She might be indifferent. Sakura could simply act like Sasuke is just another acquaintance. When she sees him, she doesn't show any special emotions, just a casual nod or a brief greeting.