Nelly Dean is the one who tells the story to Mr. Lockwood. She is involved first-hand in events. In 1802, Mr. Lockwood found her in Wuthering Heights. But she hadn’t stayed at the same place in the course of the events.
We learn the story from Nelly Dean’s narration. She could be trusted as she witnessed the drama happening between Wuthering Heights and Thrushcross Grunge. Nelly was a housekeeper at Ernshaw’s House and grew up with Catherine, Hindley, and Heathcliff. When Cathy married Edgar Linton, she moved with her to Thrushcross Grunge. The maid was a courier between the houses delivering news and fulfilling assignments. For example, she brought Linton, Heathcliff’s son, to Wuthering Heights under Heathcliff’s instruction. Later, she informed Heathcliff about Catherine’s illness.
In other words, Nelly Dean follows her mistress back and forth. She plays the role of her companion: Catherine can entrust Nelly with her intimate thoughts. After Edgar and Catherine’s death, the maid moves back to Wuthering Heights with their daughter Cathy. For Hareton and Cathy, she becomes a mother whom they never had. Nelly promotes their romance and is happy to see the wedding in the end.
Nelly is loyal, not to houses but to families. Her character is embracing and caring, wishing good to everyone. But despite her possessing such good qualities, she dislikes Heathcliff and never hesitates to demonstrate it. Once, she tells him: “If I were you, I’d go stretch myself over her grave and die like a faithful dog.”