The transformation of Barry into Savitar in the comics is a result of a combination of factors. It includes his internal struggles, external influences, and a series of choices that lead him down this dark path.
Since the title mentions Barry, he's definitely one of the main characters. There might be others like his friends or mentors who help him in learning the infinite mass punch, but without reading the fanfic, it's hard to say exactly who else would be a main character.
The fanfic 'barry learns the infinite mass punch' may have a plot where Barry is in a world full of super - powered beings. He discovers the infinite mass punch as a way to level the playing field. The story could depict his initial fumbling attempts at the punch, as it's likely a very difficult technique to master. There might be internal monologues where he struggles with the physics and the sheer power required. Along the way, he might encounter other characters who are either inspired by his determination or try to stop him out of jealousy or fear of his growing power.