Who is Sarge from The Pelican Brief and what is their importance?

Sarge (real name: Milton Hardy) is old and black with white hair springing in all directions. He always wears thick sunglasses, and encourages White House coworkers to think he is half-blind. He has worked in the West Wing for 30 years, picking up information from important people who ignore him. When he learns something important enough, he leaks it carefully to Gray Grantham of the Washington Post, arranging meetings through his son, Cleve, a Washington, DC, patrolman.