What role does the harlequin have in Heart of Darkness?

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.