Well, as mentioned, Kurtz is a key figure in 'Heart of Darkness'. He represents the dark side of imperialism and the extreme of what a man can become when removed from the constraints of civilization. Marlow is important as he gives us the perspective from which we view Kurtz and the events in the Congo. In the other short stories, different characters emerge. Some are ordinary people faced with extraordinary circumstances that test their character, while others are in positions of authority who have to make difficult choices, and all of them contribute to the overall exploration of the collection's themes.
Kurtz stands out in 'Heart of Darkness'. He is a complex character who has descended into madness and savagery due to his unchecked power in the African jungle. Marlow, on the other hand, is the observer and storyteller. He tries to make sense of Kurtz's actions and the overall situation. Other characters in the collection might not be as well - known as these two in 'Heart of Darkness', but they also play their parts in exploring themes like human nature, such as the various colonizers and the native Africans who interact in different ways.