Well, the relationship could start with Harry's squinting as a sign of his initial wariness. But as the fanfiction progresses, it could lead to a more direct confrontation. Harry might corner Wormtail and force him to face his actions. This could either end in Wormtail's redemption if he decides to turn against Voldemort for real, or it could end in his complete downfall as Harry brings him to justice. In any case, Harry's squinting at the start is like the spark that sets off the development of their relationship in the fanfic.
One possible fanfic could be 'The Abduction by Wormtail' where Wormtail uses his sly skills to get to Harry. It might have him using dark magic artifacts to overpower Harry's defenses. Another could be 'Wormtail's Sinister Plan' which details how Wormtail plots every step of the kidnapping, taking advantage of Harry's moments of vulnerability.
In the fanfiction, Harry squinting at Wormtail might also be a moment of intense focus. Harry could be trying to use his Legilimency skills (if he has developed them in the fanfic world) to read Wormtail's mind. He's squinting as he concentrates, trying to break through any mental defenses Wormtail might have. It could be a crucial moment in the story where Harry's discovery of Wormtail's thoughts could change the course of events.
In these fanfics, a popular trope is the power struggle. Harry, even while kidnapped, tries to resist Wormtail's attempts to subdue him. He might use his own magic in unexpected ways. Also, the hidden motives of Wormtail are often explored. Is he really doing it for himself or is he being forced? And of course, the element of mystery is there. What does Wormtail plan to do with Harry? This keeps readers hooked as they try to figure it out along with the story.
Perhaps there's been some sort of miscommunication or misunderstanding among Peter and his friends. Maybe a villain has manipulated the situation in such a way that his friends now think Peter has betrayed them, so they've cut off all ties with him. This would create a very lonely and difficult situation for Peter in the fanfiction.
Peter could be just an ordinary person with an extraordinary love for dogs. He might be a local boy who has always been fascinated by the tower and decides to bring his furry friends along on his adventure.
Peter could be a disgruntled noble in the fanfiction. He might be someone who has been overlooked for promotions or honors by the emperor, and this has led to his resentment and desire to kill the emperor.
Again, without more context, it's hard to say exactly who Peter Braestrup is in this 'big story'. He could be an author, an activist, or someone who made a mark in a particular industry.