Although we don't see too much of Autua compared to other characters in Adam Ewing's journal, he has, perhaps, the largest impact on Ewing's life.
After making eye contact with Ewing while being whipped, Autua is later smuggled aboard the Prophetess and makes a plea to Ewing to save his life. Before Ewing makes a decision, Autua tells him his story. The too-long-didn't-read version (even though you should read it) is this: Autua went on a journey around the world, came back home, was taken into slavery, poisoned his master and escaped, but—oops—he poisoned his master's dog by accident instead and was recaptured. He then tells Ewing to save him or kill him, because slavery is a fate worse than death. If Ewing stands by and does nothing, it would be just the same as killing Autua with his own bare hands.
Needless to say, Ewing saves Autua, and he starts doing some heavy thinking about this apathy versus action conundrum Autua has brought up.
Ewing is rewarded for saving Autua, because it's Autua who saves Ewing from Dr. Goose at the end of the novel. However, rampant 19th-century racism almost kills Ewing anyway, as a desperate Autua "asked of strangers, 'Where doctor, friend?' Thrice he was ignored (one answered, 'No medicine for stinking Blacks!')" (11.14.15). Thankfully, some kind nuns take them in and nurse Ewing back to health.
We'd love to imagine these two becoming best friends, but, due to the dicey race relations of the times, that probably won't happen, even in Cloud Atlas 2: Reincarnation Boogaloo. Still, Ewing and Autua are kindred spirits. Way back when Ewing asked Autua why he was pinning all his hopes on a little white attorney from California, "[Autua] jabbed at his eyes & jabbed at [Ewing's], as if that single gesture were ample explanation" (1.10.15).
Why does Autua see Ewing as a kindred spirit? Ewing isn't of the same race, and he's not a slave. Is it just that he saw sympathy there, or something else?