Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
First Person Omniscient
Das Kapital isn't a work of fiction, so it doesn't have a narrator in the conventional sense.
But Marx is something of a narrator himself, using the first person plural (the royal we, as it's jokingly called), like in this sentence: "We mean by labour-power, or labour-capacity, the aggregate of those mental and physical capabilities existing in the physical form, the living personality, of a human being, capabilities which he sets in motion whenever he produces a use-value of any kind" (6.2).
You'll note that after using the we, Marx quickly dives into the third person—he—to start talking about what a human being does. And for the most part, Marx describes the hes and shes of the world doing this or doing that, or he describes impersonal forces, capital or labor, doing this or that.
When he does bring up individuals, of course, he doesn't hesitate to step into their heads. For example, quoting E. F. Sanderson, owner of steel mills and forges, Marx says the capitalist speaks for all capitalists (10.4.11). Maybe Mr. Sanderson was more humble. Karl certainly wasn't.