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.
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.
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.
The Trix might also be involved. Since they are on the dark side, they could either be the ones who tempt Bloom to join them or be rivals who don't want Bloom encroaching on their territory. Also, the headmistress of Alfea College could be in it, trying to figure out what to do about Bloom's situation and how to protect the other students from the possible threat that a dark - side Bloom could pose.
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.
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.
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.
It could be an old and wise hermit who lives in the forest. This hermit has seen many things and has a vast knowledge of the skills sakura wants to learn. He guides her with tough love, pushing her to her limits but also providing valuable advice when she's in doubt.
One of the main characters could be Sakura's mother. Mothers are often intuitive about their children's problems. She could be the one who eventually helps Sakura open up about her self - harm. Then, there might be a new acquaintance in Sakura's life, someone she meets randomly. This person could have had a similar experience in the past and be able to offer unique advice and support to Sakura.
Maybe Sakura's adoptive parents. Their relationship with Sakura could be explored further in relation to the discovery of her real mother. They might have some secrets of their own regarding how they came to adopt Sakura or they could be in conflict with Sakura as she tries to find her real mother.
Definitely some of the Akatsuki members. Itachi could be one. He is a complex character with his own motives. His presence could add a lot of mystery to the story as Sakura might try to figure out his true intentions. Another could be Deidara. His explosive personality and love for destruction could create some intense confrontations with Sakura.