Mercury, also known as Merc, is a Tracker. Merc is the guy behind the majority of Faith's story. After catching Faith in his apartment approximately three years ago, Merc began training Faith as a Runner, a very special one. Acting behind the scenes Merc provides the runners with their missions and monitors the police chatter to minimize several of the encounters.
For a large majority of the game, Merc guides Faith through the different chapters, providing intel and hints as to what to do next.