Heathcliff enters the novel mysteriously, a gypsy urchin brought home by Catherine's father. Heathcliff is his only name (and it matches the windswept, bleak terrain of the Earnshaw estate, Wuthering Heights). He and Catherine form an instant bond, and when their father dies, Catherine's cruel brother Hindley contrives to keep them apart, debasing Heathcliff as a servant, and the difference in their social station drives Catherine to marry the handsome, effete Edgar Linton. Yet, Catherine maintains that she and Heathcliff are so connected to one another that they share a soul:
"This is nothing," cried she; "I was only going to say that heaven did not seem to be my home; and I broke my heart with weeping to come back to earth, and the angels were so angry that they flung me out, into the middle of the heath on the top of Wuthering Heights, where I woke sobbing for joy. That will do to explain my secret, as well as the other. 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; so he shall never know how I love him; and that, not be cause he's handsome, Nelly, but because he's more myself than I am. Whatever our souls are made of, his and mine are the same, and Linton's is as different as a moonbeam from lightning, or frost from fire.
Separated from Cathy, Heathcliff vows to wreak havoc on the external forces he perceives to be at fault, namely Hindley Earnshaw and Edgar Linton. He doesn't dare while Catherine is alive, but the tensions between Heathcliff and Edgar push her into madness and illness, and when she dies his cruelty becomes extreme:
He seized, and thrust her from the room; and returned muttering, "I have no pity! I have no pity! The more the worms writhe, the more I yearn to crush out their entrails! It is a moral teething, and I grind with greater energy, in proportion to the increase of pain.
The second half of the book--the better half, I think, though not the one that seems to be best remembered--details Heathcliff's revenge. He becomes a tenant at Wuthering Heights, knowing that the near-mad Hindley would not refuse his money, and swindles him out of the property by buying up the mortgage to support Hindley's alcoholism. He marries Edgar's sister Isabella, whom he hates and abuses, and fathers a sickly, irritable son, Linton Heathcliff, whom he contrives to marry Cathy and Edgar's daughter, Catherine. Knowing that Linton will die young--and doing his part to help!--and that Edgar's home Thrushcross Grange will pass to him, Heathcliff becomes the owner of both of his rivals' estates.
It is difficult to describe how cruelly Heathcliff goes about this. He cares about no one but the dead Catherine, and terrorizes everyone else, including his own wife and son. And yet Heathcliff and Catherine's relationship is the most spiritually powerful thing in the entire novel. Attempts to cast Heathcliff's actions in a moral--specifically, Christian--light nearly always come off as weak, and God never seems nearly as powerful as Heathcliff. As the young Heathcliff says to his nurse Nelly, "God won't have the satisfaction that I shall" when Hindley is punished. It is difficult to call Heathcliff a villain because the moral and spiritual compass of Wuthering Heights is centered, like everything else, on his relationship with Catherine.
In the end, the only thing that saves the two houses from utter destruction is a sort of spiritual elevation that Heathcliff experiences, whereby he feels himself closer to Catherine, and therefore also to death, and disinterested in what remains of an earthly world:
"It is a poor conclusion, is it not," he observed, having brooded a while on the scene he had just witnessed. "An absurd termination to my violent exertions? I get levers and mattocks to demolish the two houses, and train myself to be capable of working like Hercules, and when everything is ready, and in my power, I find the will to lift a slate off either roof has vanished! My old enemies have not beaten me--now would be the precise time to revenge myself on their representatives--I could do it; and none could hinder me--But where is the use? I don't care for striking, I can't take the trouble to raise my hand!"
This is horrible, but it is right: We could imagine the entire territory of the novel being wiped off the face of the earth by a sweep of Heathcliff's hand, but the same intensity of love that sparked his anger calls him away from it.
Wuthering Heights endures because it is like a funhouse mirror toward our most worn ideas about romantic love: It leads us toward destiny; it elevates us spiritually; it is more powerful than what surrounds it; it will survive despite all obstacles. Here, in the earthly realm at least, all these things are true, but they make no one happy, not even Cathy and Heathcliff.