In Stephen King's world of Romance there are two types of main characters, the alazons (impostors) and the eirons (the self-deprecators). Throughout most of the story, the alazon wields the upper hand but, in the end, he loses, and the eiron is restored.
The Warden and his toadies are the alazons. They are the impostors: those who pretend to be better than they really are. It looks like they are on the side of justice, but they are not. The Warden forever quotes scripture, but his words are empty; his actions reveal that he exploits the prison for profit and tortures the prisoners to satisfy his cruel form of Old Testament vengeance.
Andy is the erion, self-deprecator. He is humble, soft-spoken, and at the service of his friends. He is really better than he appears to be. He loses many battles (with the Sisters, with the library), but he wins the war because he lets the Warden underestimate him. His fight for justice is won when he escapes and exposes the Warden and Hadley as the impostors they are.
It seems that the question could use a bit more explanation. There is a definite fight for justice in King's work. The protagonists in this battle would be Andy, who has truly suffered a legal injustice and is wrestling through the moral injustices endured, as well as the overall institutional framework of prison which seeks to take everything from its inmates. Recall how Brooks is an "institutional man," in that he is incapable of being able to adapt to the outside world. This could be where the fundamental fight for justice happens. The forces of inertia that block hope and redemption are pitted against individuals who fight for such elements. Red might be pitted in the ultimate tragic condition, poised in between at first not entirely wanting to embrace Andy's belief in the power of hope and also being a man of the institution, knowing that the better part of his life has been spent inside the walls of Shawshank. Red, Andy, and Brooks represent individuals who embark on their fight for justice.