Orwell's Down and Out has no single character who is the antagonist. Instead, the enemy the narrator and those around him fight is poverty—and a social system that stigmatizes the poor as less worthy than other people. As Orwell states:
It is curious how people take it for granted that they have a right to preach at you and pray over you as soon as your income falls below a certain level.
By joining the poorest of the poor in both Paris and London, Orwell is able to document first hand that the poor are people just like the rest of. As he says:
The mass of the rich and the poor are differentiated by their incomes and nothing else, and the average millionaire is only the average dishwasher dressed in a new suit. Change places, and handy dandy, which is the justice, which is the thief? Everyone who has mixed on equal terms with the poor knows this quite well.
Second, Orwell shows in an up close and personal way the toll poverty takes on people. Be it through pawning one's clothes, working in the kitchens of Paris hotel, or tramping around the English countryside with the British poor, the antagonist the narrator fights is always poverty and the pressing needs that accompany it.