Marlow and Kurtz go together like peanut butter and jelly—up to a point. The novel depicts Marlow's slow decay and transformation into the corrupted Kurtz, but stops one vital step short: he stays sane. What's the difference? As far as we can tell, only that Marlow actually leaves. If he'd stayed in Africa, who's to say that he wouldn't have ended up putting his own native army's heads on sticks?
Like Kurtz, Marlow shows a certain amount of respect for the native Africans—admiring their physical strength and sympathizing with their plight, especially at the Outer Station. But this isn't exactly respect as we understand it today. Neither guy sees the native Africans as the equals of white men, and Marlow buys in to all the nonsense about "civilizing" them.
The helmsman isn't really a foil to Kurtz, but their deaths definitely do share some odd parallels:
These are small and insignificant on their own, but they start to build us a picture for the more important stuff—like this: Marlow says of his dead helmsman that "he had no restraint, no restraint—just like Kurtz—a tree swayed by the wind" (2.30).
But, as with any good foil, the death of the helmsman isn't identical to the death of Kurtz. When Kurtz dies, he gives us his famous, "The horror! The horror!" (3.43). But the helmsman? Marlow tells us that "it looked as though he would put to us some question in an understandable language; but he died without uttering a sound" (2.23), although he does frown menacingly. It seems like the only difference between them is that Kurtz can speak—he can give voice to the horror that both black and white men know.