A harlequin is a clownish figure dressed in colorful patchwork clothing, and that is how the man known as the Russian dresses--thus his nickname in Heart of Darkness by Joseph Conrad.
The Russian is young, twenty-five years old, and he looks even younger, as he is blond and without a beard.
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.
The Russian talks a lot now because he has lived alone the past few years and is making up for lost time. Marlowe is annoyed but lets him talk because he has so much valuable information to share about Kurtz. The Russian constantly says that Kurtz "has enlarged my mind," which makes him sound like some kind of an adoring student to the mystical and powerful orator, Kurtz.
In truth, the Russian does share some interesting bits of information with Marlow. He makes it clear that the tribe of Africans which follow Kurtz are completely dedicated to him; that Kurtz was the one who, in order to scare the steamer away, ordered the attack; and that one time the Russian was almost killed by Kurtz because of a little bit of ivory Kurtz had. The Russian shares that Kurtz loves to wax eloquent on many subjects, including (ironically) love.
The Russian owns Towson's Book and only wants to be able to live in peace here in the wilderness, asking nothing of it but the opportunity to live here peacefully. Marlow is a little perplexed by the Russian, admiring him for remaining unscathed and youthful in this harsh and devouring environment.