Cathy Earnshaw is the younger sibling of Hindley, and is born and raised at Wuthering Heights. She becomes the foster sister of the orphan, Heathcliff, at the age of six, and the two become close companions. They are separated when Hindley becomes jealous of his father's affection towards Heathcliff and reduces him to servant-boy status after the death of Mr Earnshaw, who took Heathcliff in as a Liverpool foundling. Cathy and Heathcliff's strong characters do not part them; rather, they get into a great deal of mischief together, most notably while spying at Thrushcross Grange, the fancy home of the wealthy Linton family. When a dog from the Grange attacks Cathy at her intrusion, the Lintons aid her by keeping her at the Grange for five weeks. This visit allows Cathy to turn into a lady quite unlike the rude, wild, childish girl she has been with Heathcliff, and allows her to form intimate relationships with Edgar and Isabella Linton, the two children residing at the Grange, although her (and Heathcliff's) initial impression of them was contemptuous. Cathy's change is visible on her return to the Heights at Christmas time. Heathcliff, although hurt by this, remains devoted to her, forming one part of a love triangle that includes Edgar Linton, who quickly becomes a despised rival.
Cathy's most famous scene in the novel sees a memorable declaration of her feelings for Heathcliff and Linton to Nelly Dean, the housekeeper of Wuthering Heights and the novel's main narrator:
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 because 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.
Heathcliff, eavesdropping outside, hears only that she feels that a marriage to him would "degrade" her. Immediately he embarks on a mysterious three-year absence.
This decision can be regarded as the beginning of Heathcliff's revenge on the Lintons. He later returns, a wealthy and distinguished gentleman, to find Cathy married to Edgar and living at Thrushcross Grange. The moment of his return is a telling one. After Cathy runs outside to greet him,
Mr. Linton walked to a window on the other side of the room that overlooked the court. He unfastened it, and leant out. I [Nelly] suppose they were below, for he exclaimed quickly: "Don't stand there, love! Bring the person in, if it be anyone particular." Ere long, I heard the click of the latch, and Catherine flew up-stairs, breathless and wild; too excited to show gladness: indeed, by her face, you would rather have surmised an awful calamity.
In an awkward set of visits to the Grange, Heathcliff begins to exact his revenge, seducing Isabella Linton in order to gain control of Thrushcross Grange at Edgar's death, and trapping her in an abusive and terrifying marriage. Cathy falls into a state of psychological insanity, although it is partly feigned in her desire to provoke her husband and "break his heart" because of the pain that she feels. Soon she refuses to eat, never leaves her chamber and falls prey to countless delusions and declarations of madness.
It's a rough journey, and a sad heart to travel it; and we must pass by Gimmerton Kirk to go that journey! We've braved its ghosts often together, and dared each other to stand among the graves and ask them to come. But, Heathcliff, if I dare you now, will you venture? If you do, I'll keep you. I'll not lie there by myself: they may bury me twelve feet deep, and throw the church down over me, but I won't rest till you are with me. I never will!
Heathcliff and Cathy share one final meeting, about halfway through the story, which is aided reluctantly by Nelly because of Edgar's banishment of Heathcliff from the Grange. The lovers pour out their passions to one another, and Heathcliff laments that he cannot live when "his soul is in the grave". However, when Edgar walks unexpectedly through the door to the chamber, Cathy experiences a state of shock and faints. She dies a couple of hours after giving birth to a daughter, also named Catherine, whose generation forms the basis of the second half of the story.
Cathy's spirit lives throughout the novel. Her ghost haunts Heathcliff up to his mysterious death, and an iconic scene sees Lockwood, the first narrator in the book, visited in eerie, Gothic fashion by her ghost as a little girl, lost on the moors. In Lockwood's vision she tries to enter the house through a window; at the end of the novel Heathcliff, having become desperate to see his lost love again, is found dead before an open window. The open window is therefore a symbol of Catherine's enduring power throughout the course of the story, and of her ultimate reunion with her love; however, it also raises ambiguities as to the nature of the reunion.
Cathy is said in the book to be pretty, with, as Nelly says, "the bonniest eye" and "the sweetest smile." She has long locks of "beautiful" brown hair, as Heathcliff describes it, but it is her eyes that can be seen in many characters in the novel. The dark brown "Earnshaw eyes" belong not only to Cathy, but also to her brother Hindley, and her nephew Hareton. Her daughter, Catherine Linton, inherits only two things from the mother, as we are told by Nelly: those peculiar eyes and an expression that makes her seem "haughty".
Cathy is wilful, wild, passionate, mischievous and, as a child, spoiled. As may be seen by the markedness of her change after her five-week stay at Thrushcross Grange, she is anything but a lady during her time on the moors with Heathcliff. Nelly claims that she "didn't love" Cathy, perhaps because of the girl's waywardness throughout the book. During her fatal illness, Nelly notes that Catherine is very frail, and has "a bloodless lip", an image which serves to augment the Gothic undertones of her final days; nevertheless, Nelly describes her in death as divine: "no angel in heaven looked as beautiful as her", and her countenance resembled "perfect peace".
Cathy delivers many of the lines which have become synonymous with the work, such as her renowned declaration of love for Heathcliff —
My love for Linton is like the foliage in the woods. Time will change it, I'm well aware, as winter changes the trees — my love for Heathcliff resembles the eternal rocks beneath — a source of little visible delight, but necessary. Nelly, I am Heathcliff — he's always, always in my mind — not as a pleasure, any more than I am always a pleasure to myself — but as my own being — so, don't talk of our separation again — it is impracticable.
— and the famous ghostly utterance "Let me in your window - I'm so cold!", was later used by Kate Bush in her 1978 hit "Wuthering Heights". The entertainment world, indeed, has been so intrigued by the love between Catherine and Heathcliff that many film adaptations of the novel, particularly the 1939 version with Laurence Olivier and Merle Oberon, cover only half of the story, ending with Catherine's death rather than telling the story of the younger Cathy, Hareton and Linton Heathcliff. Thematically, Catherine is also central to the issues of gender conflict, class division and violence in Wuthering Heights, as well as to the antitheses of good and evil, and reality and fantasy, which pervade the novel.