A brief search of the term foil might render this definition:
characters whose actions and traits contrast each other in a way that gives us a better understanding of both of their characters.
One asks, who is a lesser character? Anyone but Heathcliff and Catherine, really. While it might be difficult to differentiate the VOICE of the various narrators--Nelly, Lockwood, Isabella, etc--they have "contrasting actions and traits" that help readers understand the central characters. You might build a response exploring the way Nelly reacts to both Heathcliff and Catherine, to give them greater relief(i.e. definition, resolution).
Good luck! I'll take questions.