Sam's the scout who first discovered Roy and took him to Chicago for the tryouts. He's a bad alcoholic who's ruined his real scouting career with all his drinking. He's reduced to tramping around back country roads on his own, looking for the next big thing. But he cares about Roy and acts as a father figure in Roy's first fateful trip on the way to his baseball career.
It's Sam that gets Roy into the pitching/batting contest with the Whammer, all in an attempt to get Max Mercy's attention. Unfortunately, this ends up getting Sam killed (it seems that a hard pitch he took in the chest did him in) and also gets Harriet's attention, which almost gets Roy killed. With his dying words, Sam tells Roy to take his wallet and spend the money on a hotel so he can make it to Chicago for his tryout.
Sam seems to represent Roy's youthful, hopeful past. He shows up as a ghost toward the end of the novel and tells Roy, "Don't do it." We don't know what it is that he's not supposed to do until Memo and the Judge show up with their offer to throw the big game. Roy breaks his promise to Sam, leaving his youthful ideals in the dust.
Sam's kind of like Roy's Jiminy Cricket but since he dies so early in the novel Roy's on his own in the conscience department.