We first see Catherine Earnshaw through a brief glimpse at the pages of her diary – "detached sentences [. . .] scrawled in an unformed, childish hand" (3.4). Soon after, we meet her ghost, longing for reentry into Wuthering Heights (3.7). So from the beginning, Catherine is surrounded by mystery. Unlike Heathcliff, we never meet her; she died long before the story begins.
All that we come to know about Catherine is filtered through Nelly Dean, who, surprisingly, is not that much older than Catherine. Though Nelly tends to Catherine up until her death (and then takes care of her daughter, the second Catherine), she doesn't always discuss Catherine with great affection. Apparently, Nelly is really the only character ever to try to set Catherine straight, as when she questions Catherine's absurd logic about marrying Edgar. Often Nelly admits to extreme irritation with the young woman. Through the course of the novel, we come to know Catherine as an unruly and adventurous rebel, and the only Earnshaw besides her father to give a lick about Heathcliff. But Brontë doesn't make her simply the nature-loving wild child Lockwood reads about; Catherine is also a status-conscious social climber whose marriage destroys Heathcliff.
There are basically two sides to Catherine: Catherine Earnshaw and Catherine Linton. (She also fantasizes about a third, Catherine Heathcliff [3.3] – which her daughter will eventually become.) These two Catherines are very different: one is Heathcliff's Catherine and the other is Edgar's. But even when Catherine Earnshaw becomes Catherine Linton, she still maintains traces of her former self. Heathcliff longs Catherine Earnshaw; her decision to marry Edgar means that she and Heathcliff will never be together, as they were as children. Catherine's choice of husband is the pivotal choice of the novel, changing everyone's destiny and bringing the two houses – the Grange and Wuthering Heights – together.
During her weeks of recovery at Thrushcross Grange, Catherine is made into a groomed and civilized young lady. She returns to Wuthering Heights a true prima donna. This is the future Catherine Linton: a privileged and indulged lady of the house.
At one point, Nelly explains how the doting Edgar almost fears Catherine, never wanting to ruffle her feathers or contradict her. Despite his higher social status, Edgar idolizes Catherine. Her beauty and unruliness appeal to him because they are so unlike everything he has known. And let's not forget that they become an official couple after she hits him – kind of sets a tone, doesn't it? Edgar even lets the despised Heathcliff lurk around the Grange after the marriage just to keep Catherine happy.
Now Heathcliff never really calls Catherine out on her behavior either, although his obsession with revenge does indicate that he has motives that reach beyond fulfilling her needs. Still, the memories of their shared rebellious childhood recorded in Catherine's "journal" are some of the only moments of true friendship, unity, and intimacy in the novel. The novel's early pages depict Catherine and Heathcliff's childhood affections and their efforts to survive Hindley's raging abuses and Joseph's mad rantings. Roughly the next 250 show the two obsessed by a haunting nostalgia about those few fleeting moments of joy.
Everything changes when Catherine marries Edgar: not only does she commit romantically to another man, she also leaves Wuthering Heights and raises her social status far beyond Heathcliff's reach. While the essence of their love does not change, its structure and appearance do. Catherine believes that with Edgar's money she can help Heathcliff get out from under Hindley. Heathcliff sees the marriage as a rejection of him and an embrace of an entirely new life. On her deathbed, Catherine raves about the idyllic moments with Heathcliff that are now far in the past.
Though Catherine is important to the story (after all, Heathcliff is willing to die for her), she's only around for about half of the novel. She is more of a ghost, a fixation, and a memory than a character we get to know well. Buried between Edgar and Heathcliff, Catherine is in death, as she was in life, stuck between two lovers. In the end, which man was she more loyal to?