Mr. Richter is a very minor character in this novel whom we never actually meet except through the reminiscences of Oskar's grandfather as he writes about his memories of his life for his son whom he has never met. When Oskar's grandfather's account begins in the novel, he explains the process that led him to become mute and how he lost words one by one, so that he has to use notebooks to communicate with people, finding phrases that he has written before in order to converse with those around him. Into this narrative, Mr. Richter, Oskar's grandfather's "only friend," is introduced.
Mr. Richter, from what we are told about him, seems to be a character who is trying to move Oskar's grandfather on in life and to help him deal with the weight of his past. Consider how he tries to encourage Oskar's grandfather to build another sculpture. Also, note what we are told about their trip to the zoo together:
I remember spending an afternoon with Mr. Richter in the Central Park Zoo, I went weighted down with food for the animals, only someone who’d never been an animal would put up a sign saying not to feed them, Mr. Richter told a joke, I tossed hamburguer to the lions, he rattled the cages with his laughter, the animals went to the corners, we were determined to ignore whatever needed to be ignored, to build a new world from nothing if nothing in our world could be salvaged, it was one of the best days of my life, a day during which I lived my life and didn’t think about my life at all.
Mr. Richter, as another immigrant in America, seems more determined to "build a new world from nothing" in their new country compared to Oskar's grandfather. Apart from such references, Mr. Richter is a character who remains detached from the central narrative.