Nelly is certainly not reliable. She makes her dislike of Catherine quite clear and even when Catherine does deserve some sympathy (such as when she's worried about her decision to marry Edgar Linton despite not truly loving him), she rarely gives it to her.
As one of the previous answers mentions, Catherine might come off as a more likable character through someone else's eyes. After all, Heathcliff loves her (though he's no angel himself) and Edgar thinks her charming. Nelly does admit that during the period where Heathcliff is not around and Catherine is first married to Edgar, "she behaved infinitely better than I dared to expect." However, she quickly assumes this is Catherine being two-faced, whereas for all anyone knows, this is Catherine trying to forget what she has lost.
However, one thing to be said of Nelly is that she seems aware of her bias. When discussing Heathcliff's first night at Wuthering Heights , she admits she was cruel in leaving him on the stairs to sleep. She even...
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