Dr. Steven M. Safyer, the man behind the largest hospital system in the Bronx remains largely unknown, by choice. But behind the scenes, where he holds court with political and business leaders while speaking in the language of a community activist, Dr. Safyer has become one of the most powerful figures in a borough of 1.4 million residents facing a growing health crisis from obesity, diabetes, asthma and chronic diseases. So when state and city health officials wanted to save a bankrupt Bronx hospital in Westchester Square, they asked Montefiore to step in to continue providing services.