Nostromo symbolizes the People -- the great mass of mankind who are used and abused by the mighty forces of politics and history, the ones who are acted upon by the various dream-ideals of every competing faction, which would in turn be powerless without them. "Invaluable fellow," Mitchell's casual aside at Nostromo's first mention in the novel, is actually a loaded phrase. The remarkable word "fellow" can mean (in addition to Mitchell's slang use) either "leader," "equal" or "lower-class person": the perfect sliding sobriquet for the all-powerful yet ever-victimized People.
Again, when Mitchell calls Nostromo "a fellow in a thousand," he
means to imply Nostromo's superiority, yet the phrase also subtly equates him
with the throng.