Well, we are certainly given an insight into how important the "helmsman" was to Marlow when he is killed just as they are drawing into the Inner Station to meet with Kurtz and the natives fire a storm of arrows at the steamer and a spear is thrown that kills the helmsman. Marlow explains the grief he feels at his passing:
I missed my late helmsman awfully - I missed him even while his body was still lying in the pilot-house. Perhaps you will think it passing strange this regret for a savage who was no more account than a grain of sand in a black Sahara. Well, don't you see, he had done something, he had steered; for months I had him at my back - a help - an instrument. It was a kind of partnership.
Marlow talks of how this "bond" was established and how, even now, he is able to remember the "intimate profundity" of the look he gave Marlow when he died. Having worked alongside Marlow for so long, Marlow now feels his absence, and this explains why he throws the body into the river so that the corpse is not eaten by the other natives on board.