Wuthering Heights utilizes a somewhat complicated frame story. It revolves around Lockwood, the new tenant at Thrushcross Grange, gradually learning the history of his landlord Heathcliff at neighboring Wuthering Heights and Heathcliff's connection with the Lintons of the Grange. Lockwood learns this story from former Wuthering Heights servant Nelly Dean, who now works at the Grange and keeps Lockwood company while he's sick. In effect, then, Nelly narrates most of the story.
Nelly is the perfect narrator for the frame story. As a lifelong servant, she's intimately familiar with the Earnshaws, Lintons, and Heathcliffs, yet, in terms of class, she's a perpetual outsider. Though it's mentioned a few times that Nelly is fairly well-read, and that's reflected in her speech, she's always been a household employee, and that means there's an uncrossable boundary between her and her employers' families.
Roughly in her 40s in the novel's present, Nelly has known most of the characters since they were children; she grew up with Hindley and Catherine Earnshaw, nursed young Hareton Earnshaw, and helped raise Cathy Linton, so she's very sympathetic to her former charges in particular, and mistrustful of Heathcliff because of his antipathy toward them. And because of her role as confidante to young Catherine and now Catherine's daughter Cathy, she has inside knowledge of the young women's feelings about the young men over the years. Nelly clearly has her biases, and the novel doesn't explore whether she editorializes to any significant degree; given her extremely detailed narrative, modern readers would assume she does, but that simply isn't a concern of the novel. Rather, her constant presence, protective loyalty, and observant, opinionated nature make her an ideal character to relay Wuthering Heights' history to Lockwood—she belongs in its history without being completely of it.