There are a lot of side characters, and each has a role to play. Here's a couple that come to mind.
The Aunt is important because she helps set the plot in motion, and because she represents the common feeling of the time towards colonization: the Aunt speaks of missions to Africa as "weaning the ignorant millions from their horrid ways."
The Chief Accountant is interesting because he maintains his poise and bearing even when people are dying all around him. Marlowe says "That's backbone," when describing him. One could say he represents and ideal that most cannot achieve.
Captain Fresleven is Marlow's predecessor and started off as a normal man. However he argues with a village chief, beats him, and is later killed. This is to illustrate how Africa can make a European man mad if he is not careful.