Who is Admiral Greer from Clear and Present Danger and what is their importance?

Admiral Greer is the current Director of Intelligence for the CIA. It was he who brought Jack Ryan into the agency and he was also responsible for the recruitment of Dan Clark. Immensely popular in the agency, it is not only Jack Ryan who finds a surrogate father figure in him. Unfortunately he is hospitalized in Bethesda Hospital with pancreatic cancer. It is Admiral Greer who admonishes the man who is replacing him to be aware of all his responsibilities and his duty to inform Congress.