What role does the harlequin have in Heart of Darkness?
The harlequin aids in the characterization of Kurtz. Without him, Conrad would have had to find another way for Marlow to come to a fuller understanding of Kurtz's psyche before he meets him himself. The fact that the harlequin has become a disciple of Kurtz more deeply informs Marlow that Kurtz is a Svengali of sorts, a figure who seduces and manipulates those who enter his orbit.
The harlequin, known traditionally as a "fool," also embodies the proverbial idea that "fools rush in where angels fear to tread," or that in one of life's often observed ironies, fools survive. In some ways, his youth and susceptibility function as a foil to Kurtz's age and craftiness. Marlow's observation of the harlequin is that he is young, and readers understand that his youth is at least partly responsible for his inability to recognize Kurtz as the megalomaniac he has allowed himself to become.
The harlequin, or Russian, gives Marlow with a great deal of information about Kurtz, the he would not have had without the Russian's presence. The Russian most important information is that Kurtz is adored by the African tribe that follows him, that he once nearly killed the Russian for his small supply of ivory, and that it was Kurtz who ordered the attack on the steamer to scare them away. He is also a person who is unconditionally loyal to Kurtz, even saying "This man [Kurtz] enlarged my mind.