Mr. Hindley came home to the funeral; and—a thing that amazed us, and set the neighbours gossiping right and left—he brought a wife with him. What she was, and where she was born, he never informed us: probably, she had neither money nor name to recommend her, or he would scarcely have kept the union from his father. (6.1-2)
Frances joins the unwelcoming Earnshaw clan. Though unknown and without family or fortune (just like Heathcliff), she has managed to win Hindley's affections. Curiously, this is one of the only mentions of neighbors. Who knew there was anyone else out there on the moors?
He drove him from their company to the servants, deprived him of the instructions of the curate, and insisted that he should labour out of doors instead; compelling him to do so as hard as any other lad on the farm. (6.9)
Hindley's project to punish his father's favorite begins as soon as the old man dies. To make Heathcliff a farmhand, bereft of education (instructions), is to put him in the lowest possible position. The gentry never work with their hands.
[. . .] instead of a wild, hatless little savage jumping into the house, and rushing to squeeze us all breathless, there 'lighted from a handsome black pony a very dignified person, with brown ringlets falling from the cover of a feathered beaver, and a long cloth habit, which she was obliged to hold up with both hands that she might sail in. (7.1)
After staying at Thrushcross Grange, the untamed Catherine has become a changed woman, now superior to the lowly Heathcliff. This is the future Catherine Linton, now forever out of reach to Heathcliff.
Ellen "Nelly" Dean
"Were I in your place, I would frame high notions of my birth; and the thoughts of what I was should give me courage and dignity to support the oppressions of a little farmer!" (7.44)
Since he doesn't know where he is from, Heathcliff may as well imagine a noble and exotic background for himself. This piece of advice represents one of a handful of Nelly's attempts to provide useful guidance for Heathcliff. It also tells us that she likes a little fiction.
[Hindley] wished earnestly to see her bring honour to the family by an alliance with the Lintons, and as long as she let him alone she might trample on us like slaves, for aught he cared! (9.152)
Hindley has designs on the Lintons' social status. Nelly resents the treatment she receives from Catherine. Nelly (who is speaking here) may not be a slave, but she is a servant—yet more often than not she acts like a family member.
"I've no more business to marry Edgar Linton than I have to be in heaven; and if the wicked man in there had not brought Heathcliff so low, I shouldn't have thought of it. It would degrade me to marry Heathcliff now." (9.92)
Catherine realizes that Edgar is out of her league, but that doesn't stop her. As a child she ignored everyone else's dislike of Heathcliff, but now she allows Hindley's attitude and treatment of him to change how she feels. In that sense, Hindley really gets what he wants.
A half-civilized ferocity lurked yet in the depressed brows and eyes full of black fire, but it was subdued; and his manner was even dignified: quite divested of roughness, though stern for grace. (10.53)
Though still swarthy, Heathcliff is a changed man. Gone for three years, he returns with some grooming and social graces. Clearly he has been working hard on improving himself—but that hasn't changed his overall attitude.
Her brother, who loved her tenderly, was appalled at this fantastic preference. Leaving aside the degradation of an alliance with a nameless man, and the possible fact that his property, in default of heirs male, might pass into such a one's power. . . . (10.82)
Heathcliff's aim to captivate Isabella torments Edgar. Because Edgar does not have a son, Isabella's marriage to Heathcliff means that Thrushcross Grange will eventually belong to the orphan outsider.
Catherine Earnshaw Linton
"Tell her what Heathcliff is: an unreclaimed creature, without refinement, without cultivation; an arid wilderness of furze and whinstone." (10.98)
Catherine's warnings about Heathcliff only stoke the fire of Isabella's desire. And, to be honest, all of the qualities she cites to get Isabella to change her mind are the very things that Catherine loves in Heathcliff.