Heathcliff is made to feel like an outsider by his own adoptive family, which fuels his desire for revenge. Wuthering Heights never makes it clear where he is originally from, although Mr. Earnshaw says he picked him up in the streets of Liverpool... a port town where immigrants entered England. A ton is made of Heathcliff's appearance; the contrast between his swarthy, brooding looks and Edgar Linton's creamy, soft skin is dramatic.
So Heathcliff is a double outsider: not only is he not related to anyone at Wuthering Heights, but he is also marked as racially different. He is described dozens of times as a "dark-skinned gipsy" (1.15). Nelly basically sums up the regional biases of the Yorkshire inhabitants when she says, "We don't in general take to foreigners here, Mr. Lockwood, unless they take to us first" (6.7). But Heathcliff's foreign appearance might partially explain why Catherine thinks he's so hot.
Questions About Foreignness and the Other
Is there any connection between foreigners and ghosts?
How does Heathcliff's racial identity affect his destiny?
What does Brontë suggest by having Heathcliff named after a dead child?
What do characters mean by calling Heathcliff a "gipsy"? What does that epithet imply?
Chew on This
Heathcliff is as much a threat for being foreign as he is for upsetting the family hierarchy as Mr. Earnshaw's new favorite.