The main characters could also be entirely new characters introduced in the fanfic. The author might create someone who has a unique connection to the events of the original series and is given the task of changing things. However, it's also possible that classic villains like Cersei could be main characters if the fanfic is about giving her a second chance at ruling, perhaps with a different approach this time.
In the 'Second Chances' 'Game of Thrones' fanfic, it might focus on different plotlines. For instance, it could explore how a minor character who was overlooked in the original series gets a second chance to shine. It could be that this character has some hidden power or knowledge that, with this new chance, they can use to influence the power struggles in Westeros. This could lead to new alliances and rivalries that we didn't see in the official story.
It's hard to say for sure without reading it, but it could be Harry Potter himself. Since it's a Harry Potter fanfic, he's likely to be involved. He might be the one getting a second chance at something important, like saving Sirius Black.
There could be a female knight from the Reach who has to fight against the prejudice in a male - dominated society. She might be on a quest to prove that women can be just as good, if not better, in combat and leadership. Also, perhaps an exiled noble from Dorne who is trying to regain his family's honor and lands through various means, including forming unlikely alliances with other outcasts.
Obviously, Harry Potter would still be one of the main characters. Since it's about second chances, he might be dealing with things he didn't get to do right the first time. Hermione Granger would also be a key character as she's always been resourceful and could play a big role in making the most of these second chances. Ron Weasley too, as he's Harry's best friend. And perhaps some characters who were on the fringes in the original, like Luna Lovegood, could step into more prominent roles this time around.
There could be a young, ambitious knight who has seen the injustices in the Seven Kingdoms and becomes a key figure in the uprising. Another might be an old and wise maester who secretly supports the uprising and uses his knowledge to help the rebels. Also, a strong - willed noblewoman who has her own reasons for wanting change could be one of the main characters.
There's no way to know exactly without reading it. However, in a 'Game of Thrones' - themed fanfic with 'no mercy' in the title, it's likely to have some of the fan - favorite characters in key roles. Maybe Arya Stark, with her assassin skills, could be a main character, showing no mercy to those on her kill list. Or it could be Cersei Lannister, being even more merciless in her quest for power and revenge than she was in the original series.
Since I haven't read this fanfic, I don't know the main characters. It could be original characters created by the fanfic author, or it could be based on characters from an existing work that the fanfic is related to.
Most likely, there will be the traditional Stark family members. Ned Stark's children like Arya, Sansa, Bran, and Rickon could play significant roles. They might be joined by other Starks who were not as prominent in the original series but are given more spotlight in this fanfiction.
I don't know for sure as I haven't read it. But it could be Ruby, Weiss, Blake, and Yang as they are the main characters in the RWBY series and often feature prominently in fanfics.
I think Naruto would be like the central hero, similar to how Daenerys was in Game of Thrones. He wants to bring change and justice. Then there's Kakashi, who could be like a wise advisor, much like Tyrion Lannister in some ways. And Gaara, with his past of being an outcast, could be a character who rises above his circumstances, just like Theon Greyjoy did at some points.