Brontë uses multiple, overlapping narrators—chiefly Nelly and Lockwood—to convey different various attitudes about her characters, creating a tone that ranges from detached fascination (Lockwood) to maternal sympathy (Nelly).
The frame story, with Nelly relating Wuthering Heights's history to Lockwood, is the novel's most potent tool for tone-setting. Nelly grew up at Wuthering Heights and knows most of the characters intimately, yet she's also distanced from the conflict due to her servant status. She therefore effectively sets the novel's tone regarding its characters: she has both a maternal and sisterly role in Catherine's life and thus has a sympathetic yet frequently exasperated attitude toward her; she helped raise Hareton and Cathy, so she's tender and protective of them even when she reproves them for being rude or prideful. And, unsurprisingly, she's suspicious and fearful of Heathcliff because of his treatment of her beloved charges. Yet because of her toughness and their long history, she's able to stand up to Heathcliff, and he respects her enough to be honest with her about his intentions—or as honest as he is with anybody. So, while Nelly is clearly biased, she also has the most realistic, believable perspective in the novel.
The novel also conveys an ironic tone through Lockwood's early observations, and even disgust when he first gets acquainted with the people of Wuthering Heights—they're so uncivilized he can hardly take in what he's seeing at first (hence his horrified use of verbal irony when describing characters); yet, afterward, he's so curious that he keeps pestering Nelly for stories about Wuthering Heights and thereby tugs the reader along, too. By the very end of the novel, Lockwood returns for a visit and has a far more benign and hopeful tone toward the surviving family at Wuthering Heights, suggesting that their future will be much happier than their fraught past and permitting the reader a sense of resolution, too.