Ryabinin is the land merchant who successfully cheats Oblonsky. For Levin, Ryabinin's cheating of Oblonsky represents a problem of the decaying nobility in Russia. As peasants are growing richer, the aristocracy is growing poorer. According to Levin, this would be fine, except that what is making the nobility poor is not lack of ability, but a lack of knowledge. As a class, they're ignorant, and therefore vulnerable to immoral types like Ryabinin.