Old Misery comes to symbolize the hardship in the aftermath of WWII. He's portrayed as living in a damaged house with broken plumbing and electricity. The house used to be beautiful, refined, and upper class, but now, because of the war, it stands alone in a bombed out neighborhood. Old Misery, like his house, experiences isolation and subsistance living, in the aftermath of the war.
I think Old Misery is used by Greene as a sympathetic character who the reader definitely comes to like, and by the end of the story, pity. This heightens the shock of the destruction that the boys commit by destroying his house. Greene is careful to paint a man who is lonely, does his best to understand the boys, and is hungry for companionship. The destruction of his house at the end of the story, and in particular the laughter of the driver, exacerbates the nihilism that is central to the tale and in particular the character of T.
Old Misery is a character added to break tension and suspense during the story. He is naive about the ways of the boys. He adds a bit to the humour in the story, thereby, lightening the gloomy mood that prevails throughout the story. He is the only beacon of hope, and strives to remain so, and Greene uses him and the boys to describe the never-ending conflict of gloom, fear, darkness againt hope, kindness and light.