Davis is the captain of the Ipecacuanha, the ship that found Prendick adrift in the ocean. He's a rough-tempered jerk who consumes alcohol like the rest of us take in air. Seriously, we don't think there's a scene where this guy isn't blitzed out of his skull.
Although he rescues Prendick initially, he soon grows irritated with him and eventually casts Prendick back into the ocean to die. Unsurprisingly, Prendick thinks this is a bad idea, maybe even an immoral one. Davis says he's well within his rights to do so, exclaiming "Law be damned! I'm king here" (5.22). Through Davis we get a sense that Prendick has left the traditional rules and order governing what we'd consider civilization. Of course, things only get weirder as Prendick travels into Moreau's little world.
On a side note, Davis may or may not be one of the corpses Prendick discovers in the dinghy at the end of the novel. It's left ambiguous on purpose. What do you think: is it Davis? Also, we're curious; would it change your reading of the novel's ending scenes if it was or wasn't and how?
He is only mentioned in Moreau's story to Prendick. The Kanakas missionary took Ape Man under his tutelage. He taught the beast language, the Bible, and other social skills. Eventually all the Kanakas died out, but the missionary's teaching were kept by the Beast Folk society, and eventually morphed into the Law.