Character Analysis
Workers are the ordinary folks who sell their labor-power as a commodity in exchange for wages with which to consume use-values. They're labor personified.
Let's look at that in detail.
In circulation (the movement of commodities in exchange), C-M-C is the process or "circuit" of selling in order to buy. C stands for commodity, and M stands for money. Workers start off with their labor-power as a commodity (the first C) and sell it to acquire M, their wages. Then they use the M to buy commodities, the second C, such as the means of sustenance (food, clothing, shelter).
Marx's whole point with the C-M-C jazz is to describe what ordinary worker schlubs like you and me do in the market. We sell our labor-power to our employers, receive money for it, and then take the dough to the store and buy commodities to consume as use-values. We're caught in that same cycle over and over. Lather, rinse, repeat.
Meanwhile, capitalists get to engage in the M-C-M' process to acquire more money. They put their money to work; workers put their own labor-power to work and generally aren't paid enough to start accumulating surplus-value—which keeps them from becoming capitalists themselves.
Here's ol' Karl on C-M-C. "The direct form of the circulation of commodities is C-M-C, the transformation of commodities into money and the re-conversion of money into commodities: selling in order to buy" (4.4). And again: "In the circulation C-M-C, the money is in the end converted into a commodity which serves as a use-value; it has therefore been spent once and for all" (4.10).
Marx usually considers workers impersonally, as the abstract force that sells labor-power. But sometimes he depicts actual workers. For example, there's this sad image of a worker ripe to be abused: the worker "is timid and holds back, like someone who has brought his own hide to market and now has nothing else to expect but—a tanning" (6.23).
But at other points, such as in Chapter 10, Marx makes workers sound heroic as they fight for factory reforms. For example, "The factory workers, especially since 1838, had made the Ten Hours' Bill their economic [...] election cry" (10.6.10). Or, when the manufacturers attempted after the Factory Act of 1850 to use children to force the working day of adult males up to 15 hours, the workers "demonstrated that such an attempt was bound to fail in face of the resistance of adult male workers" (10.6.36).
Basically, the workers are Marx's heroes—even if they're not the main attraction in Das Kapital itself.
Worker's Timeline