What particularly does Hareton represent to Heathcliff in Wuthering Heights?
In chapter 31 of Emily Bronte's Wuthering Heights we find Lockwood making an analysis of what he is witnessing from the members of the household. There is a particular interest in the character of Hareton Earnshaw. He is a young man, messy, illiterate, but still strongly-built and with an even bigger attitude. He is treated like a servant although he seems to be an essential part of the manor. He also shows pride in his name, as if he had been part of a historically-famous family.
Yet, it is Heathcliff's behavior towards Hareton what makes Lockwood wonder what exactly is going on. We are told from the narration that Heathcliff can hardly stand the sight of Hareton, and that Hareton resembles his aunt Catherine more and more. Heathcliff also seems to feed off the dysfunctional relationship between Hareton and Catherine: Although they clearly seem to have a romantic tension going on, they choose to be mean to each other.
This being said, there is no doubt that what Hareton represents is a young Heathcliff back in the times when it was Catherine Earnshaw who would tease him. Similarly, Hareton is a messy, disheveled kid just like Heathcliff was, as a foundling. In the same way, Heathcliffhad an uncanny amount of adoration for Catherine which later turned into hatred and hunger for revenge. He lost Catherine to Hareton's father, and now he seems to enjoy seeing Hareton in the same situation.
Therefore, Heathclifftreats Hareton like he was treated when he was a child by Hareton's father, Hindley. In the same line, Heathcliff also cathartically enjoys the tense relationship between Hareton and Catherine because it mirrors the relationship he once had with the first Catherine right before she chose Hindley as a husband. This fascination is all part of Heathcliff's sick and sadistic personality: One who is so hungry for revenge that he is willing to sacrifice anything or anyone for it.