De Lacey is the Parisian-turned-blind-peasant who lives in the cottage. The monster tries to talk to him. but his children drive him away. De Lacey serves a few different purposes. First, he demonstrates the role of family in juxtaposition to the loneliness that the monster and later Victor feels. He has a son, Felix, and a daughter, Agatha, who both live with him, so he is surrounded by warm fuzzy family feelings all the time. The monster, of course, watches this with envy.
De Lacey being blind is also a huge deal. The monster’s claim that "people only hate me because I’m ugly" is essentially validated by this guy. If only everyone were blind, the world would be such a better place.