The manager is a mediocre Company employee who lives and works at the Central Station. We're thinking that he works at the "central" station because he's average and commonplace (you know, central) in every way—that is, except for his "remarkably cold" eyes and creepy smile (1.52). That expression gives Marlow the willies. (Us too.)
The manager is jealous of Kurtz's success, but other than that he's a total blank—which is the point. He babbles a lot, but about nothing meaningful and his creepy smile is described as "seal applied on words to make the meaning of the commonest phrase appear absolutely inscrutable" (1.52). In other words, all his chitchat ends up seeming profound because he slaps on this mysterious, empty, smile.
He also has one other remarkable quality—he never gets sick. Maybe, Marlow says, because "there was nothing within him" (1.52). The manager himself says that anyone who comes to work in the interior "should have no entrails" (much like him).
Weirded out yet? You should be. The manager's character implies that the wilderness of the interior has a way of depleting or draining away what makes men human, leaving only a shell of the former self. Sort of like Kurtz, except instead of being replaced by a maniacal, ivory-hungry devil, this guy got replaced by nothing at all.
Since there is nothing within him, everything the manager says and does has no sincerity. All his energy is devoted to keeping up appearances. As Marlow observes, he "originat[es] nothing" because there's nothing there (1.52). He can't create; he can only destroy.
Huh. You could say that "destroying" is exactly what British imperialists were doing to Africa at the time. We knew this character was here for a reason.