The main characters are almost surely Sasuke and Sakura. Given that it's a fanfic related to them, as indicated by 'SasuSaku'. Sakura is directly related to the event of the seal breaking, and Sasuke, being an important part of the SasuSaku relationship, will surely play a role in the story. He could be a source of support for Sakura, or there could be some conflict between them because of the seal - breaking incident.
Based on the title, the main characters are likely to be Sasuke and Sakura. Since it's a SasuSaku fanfic, they are usually the central focus.
I'm not entirely sure as I haven't read this specific fanfic. But generally, in SasuSaku (Sasuke and Sakura from Naruto) fanfics, it could be about some new adventure that breaks Sakura's seal and how that impacts her relationship with Sasuke and the overall story world.
Well, in such a fanfic, perhaps Sakura could gain new powers. With the seal breaking, she might be able to access a hidden reservoir of chakra. This could lead to her being more formidable in battle and changing the power dynamics within the Sasusaku relationship.
Obviously, Sasuke would be affected. Since it's a Sasusaku - related fanfic, any change in Sakura would impact their relationship. He might have to adjust to a more powerful Sakura, or he could be worried about the consequences of the seal breaking.
If Sakura dies in a fanfic, it could have a huge impact on the whole Naruto -verse within that story. Her friends like Naruto would be deeply affected. Naruto might feel like he failed to protect her, just as he felt about other losses in his life. And for the medical ninja community in the fanfic, her death could be a big blow as she was a talented medic.
In some cases, Karin can be more popular. Her backstory of being associated with Orochimaru and her survival instincts make her an appealing character. Some fans might be drawn to the idea of her as an underdog in the love triangle with Sasuke and Sakura. Also, her sensing abilities and her role in certain plot points in the original series can be explored more deeply in fanfics, which might make her more interesting to some readers than Sakura.
A fanfic could show Sakura sacrificing herself to save Sasuke. This would be a very emotional and heroic end for her. Sasuke would then have to live with the guilt and the memory of her selfless act.
It might explore different aspects of Sasuke and Sakura's characters within the context of a fanfic. For example, it could show Sakura with different abilities or Sasuke having a different attitude towards her. Since it's a fanfic, the author has the freedom to deviate from the original storyline and create something new. It could also be about their relationship in a different time period or setting, like in a modern - day AU (alternate universe) or in a completely fictional world.
Ino. Since she is Sakura's friend, she can bring in some girl - talk moments. She could also be a bit jealous of Sakura's relationship with Sasuke at first, but then come to accept it and even help them out in some situations in the fanfic.
It could be Orochimaru. He's always been interested in Sasuke and might see kidnapping Sakura as a way to manipulate Sasuke. He could use Sakura as a guinea pig for his experiments or as a bargaining chip to get Sasuke to come to him.
Well, one possibility could be that it's a dark fanfic where an enemy of Sakura kidnaps her to get to Sasuke. Maybe they want to use her as bait to lure Sasuke into a trap. Since Sasusaku is a popular pairing, the kidnapper might think that Sasuke would do anything to save Sakura.