Mr. Riach is the second mate of the Covenant, and yet another complex and contradictory man. He's a drunkard who gives cabin boy Ransome wine against all good sense – but he does it to be kind. He's also responsible for rescuing Davie from the hold of the Covenant, where he is dying of illness, and bringing him up to a higher part of the ship for fresh air. And while Riach is one of the ones who attacks Alan for his money belt on board the Covenant, once the ship is wrecked on the shore of the island of Mull, Riach helps protect Alan from Captain Hoseason.
Riach's motivations are pretty mysterious, even to himself. He tells Davie that he is the educated son of a laird, yet here he is second mate of a two-bit ship. Why? "I liked fun, that's all" (7.41). Now there's a motivational speech we can get behind. At any rate, to follow on this point we keep making about contradictory characters in Stevenson's work, Riach's complexity arises from a lack of self-knowledge. He doesn't seem to know why he does the things he does; he just does them. He's not just torn between his good and evil natures – he doesn't seem to be self-aware enough to know what his nature is or what he's doing.