We are formally introduced to Catherine Earnshaw in Volume I: Chapter IV.
Catherine is the youngest child of Mr. and Mrs. Earnshaw, of Wuthering Heights.
Catherine, born in 1765, is eight years younger than her brother, Hindley.
We come to Chapter IV and Ellen “Nelly” Dean—in service at Wuthering Heights since she was a child—shares details of Heathcliff’s arrival; we become acquainted with Catherine, a six-year-old child.
Ah! But, we are already familiar with Catherine. In Volume I: Chapter III, the tenant of Thrushcross Grange—Mr. Lockwood—finds Catherine’s diarying and marginalia.
When Lockwood falls asleep in what was once Catherine’s bed chamber, he dreams of the girl—in the form of a ghost-child wailing outside his window: “I’m come home, I’d lost my way on the moor!”
In those early chapters we are still getting our bearings. Lockwood is a tenant of the Grange, but must stay overnight at the Heights. Heathcliff is a morose landlord. And there is no sign of Catherine. It is through Ellen “Nelly” Dean’s narration we begin to understand the relationship between Catherine Earnshaw and Heathcliff—and we are provided all the details necessary to understand why it is not Catherine Earnshaw but instead, Catherine Linton who inhabited Mr. Lockwood’s dream.
She was much too fond of Heathcliff.

Wuthering Heights is a story of Catherine Earnshaw and a foundling called Heathcliff.