No one, that's the problem.
The problem really is that Heathcliff has no guide or mentor. Though Catherine is an ally, she's no advocate, and her dream of supporting Heathcliff on Edgar's dime is just that – a dream. Joseph is like a false mentor, with all of his pious and judgmental nonsense. Nelly Dean has too many opinions, though she is occasionally kind to Heathcliff, as when she helps clean him up and change his attitude in Chapter 7. Where Heathcliff gets his ideas for revenge is unknown. His source of inspiration is his love, in which sense Catherine is his muse, but his status as an orphan and his utter isolation are largely what define him. If he had had a guide – for instance, if Mr. Earnshaw had not died and left behind a big fat mess – things would have turned out very differently for our protagonist. But then, we wouldn't have had much of a novel.