The harlequin is a Russian man who helps Kurtz and is considered his "disciple." He dresses in colorful patched clothing, which earns him his nickname of—yes, you got it—the harlequin:
His clothes had been made of some stuff that was brown holland probably, but it was covered with patches all over, with bright patches, blue, red, and yellow—patches on the back, patches on the front, patches on elbows, on knees; coloured binding around his jacket, scarlet edging at the bottom of his trousers; and the sunshine made him look extremely gay and wonderfully neat withal, because you could see how beautifully all this patching had been done. (2.34)
He worships Kurtz much like the native Africans do and finds himself listening more than speaking. In fact, exclusively listening and not speaking at all. Thus, he's privy to many of Kurtz's thoughts. Like many of the other characters, he has a tendency to babble, but Marlow tolerates the harlequin because he knows so much about Kurtz.
The harlequin's catchphrase is that Kurtz has "enlarged my mind" (2.37). This paints Kurtz as a guru possessing arcane and mystical knowledge, and the harlequin as being, well, a little dim-witted. Plus, those funny clothes make the guy out to be something of a court jester for Kurtz, a clown not to be taken seriously. He acts only as a conduit of information for Marlow.
One last thing: the harlequin's "blue, red, and yellow" patches sound to us a lot like the map that Marlow sees when he walks into the company office:
Deal table in the middle, plain chairs all round the walls, on one end a large shining map, marked with all the colours of a rainbow. There was a vast amount of red—good to see at any time, because one knows that some real work is done in there, a deuce of a lot of blue, a little green, smears of orange, and, on the East Coast, a purple patch, to show where the jolly pioneers of progress drink the jolly lager-beer. (1.22)
Those colors represent all the parts of Africa that had been carved up in the so-called "scramble for Africa" of the late nineteenth-century. (Check out a harlequin-style map here.) But the harlequin is Russian, and Russia didn't have any African territories. So, what's the relationship between this patchwork continent and the harlequin's patchwork clothes?The Harlequin Timeline