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.
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.
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.
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.
Obviously, Sasuke and Sakura are the main characters. Sasuke with his complex past and new - found purpose after the war, and Sakura, who has always had feelings for Sasuke. In the fanfic, they are likely to be portrayed dealing with various post - war situations, like rebuilding the village, and their relationship is central to the story. They are the ones the readers will be most focused on as the story progresses.
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.
There might also be other characters involved, like Sakura. Since she was Tsunade's apprentice, she could be part of the story. Maybe she's caught in the middle when Tsunade has to take some action regarding Naruto. Also, Kakashi could be in the fanfic. He has a close relationship with Naruto and might be affected by whatever Tsunade does to Naruto or be involved in the events that lead to Tsunade's actions.
Obviously, Harry would be one of the main characters. He is the one making the decision to break the alliance, so his character development would be central to the story.
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.