Perhaps in the fanfic, Brick has some sort of relationship with Blossom that she doesn't approve of. It could be a forced romantic relationship or a partnership where Brick is constantly overshadowing Blossom or not respecting her. Another aspect could be that the fanfic changes the fundamental nature of the world they exist in in a way that Blossom finds unacceptable, and Brick is somehow a symbol of that change.
One way could be for the writer of the fanfic to rewrite Brick's character to be more palatable to Blossom. Maybe make him more heroic or less antagonistic.
Maybe in the fanfic, Blossom gets injured during a battle and Brick, instead of leaving her as an enemy, decides to help her. This act of kindness makes Blossom question her view of the Rowdyruff Boys. They then start secretly meeting up to exchange information about potential threats. As time goes on, their meetings turn into something more like dates, and they fall in love despite the odds being against them.
In the fanfic, Naruto could hate the Hokage because of some bad experiences related to past Hokages. Perhaps he discovers some dark secrets about how the Hokage office has made decisions that he doesn't agree with. It might be something like a cover - up of certain events that Naruto believes should have been dealt with more fairly.
One possible reason could be a misinterpretation of his character. In the original series, Harry Potter doesn't hate Muggles at all. But in this fanfic, perhaps the author wants to explore a 'what if' scenario where something drastic has happened to make him have such feelings. Maybe he's been betrayed by a Muggle - in the fanfic world, this could be a made - up situation that sours his view on all Muggles.
Maybe in the fanfic, there could be a misunderstanding between them. For example, Ron might have accidentally broken Harry's trust in some way, like revealing a secret of Harry's that he wasn't supposed to.
Maybe in that fanfic, there are misunderstandings between Harry and the Weasleys. For example, they might have had an argument over something like magic use or family values that led Harry to dislike them.
The writer might be trying to add some drama to the story. By making Aang hate Katara, it creates a conflict that can drive the plot forward. For instance, their friends might have to work hard to resolve this hatred and restore their friendship.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
Perhaps in this fanfiction, Brick and Blossom are transported to a parallel universe where everything is made of lemons. They have to find a way back home. In this strange world, they meet different versions of their friends and enemies. They use their combined powers and intelligence to navigate through this lemon - themed world and find the portal that will take them back to their own world.
Well, perhaps in the fanfic, Blossom and Brick are in an alternate dimension where they are the only ones with powers. They have to rely on each other to survive. Along the way, they have many arguments but also some tender moments. One day, Brick gets injured while protecting Blossom. She's so moved that she kisses him as he recovers.