Joe Keller's words provide the title of the play. His suicide provides the resolution. He's the first person we see on stage, and all the action happens in his backyard. He undergoes a big change throughout the play, first publicly denying his involvement in the machine shop scandal, then admitting yet excusing his actions, and finally accepting full responsibility for the deaths he caused. He would seem to be the protagonist, and yet he's not particularly active. He works only to defend his turf.
On the other hand, Chris has a set goal in mind, and maneuvers toward it. He will ask Ann to marry him, and somehow get his parents to accept the union. Chris undergoes a transformation almost as massive as Joe's (excepting the death part, of course). He sees his father and himself as innocent, yet eventually accepts his father's guilt and his own cowardice for refusing to confront him.