Steve Mason is a large, older, heavyset man with gray eyes. He's bald and has a thick, gray mustache. He is also always seen wearing a black suit and a tie.
In the early stage of the Kira investigation, Mason helps L by sending twelve FBI agents to Japan in order to investigate the source of a leak from within the Kira Task Force, but he withdraws support for the case after all of the agents are killed by Kira.
After the timeskip, he is approached by Near and together they convince United States President David Hoope (George Sairas in the anime) to form the SPK. Mason himself is a member of the SPK and, along with several others, is killed by the Death Note when it falls into the hands of the Mafia.