Isabella Linton, of Thrushcross Grange, is the sister of Edgar Linton; the man who will become Catherine Earnshaw's husband, much to Heathcliff's chagrin.
Isabella is a lady. She is brought up in a pampered environment with all the respect and uniqueness that is due a person of high birth and social ranking. All of this, however, will be completely juxtaposed to what her life will become not long after her elopement with Heathcliff.
Heathcliff cunningly manipulates the fact that Isabella, who has never seen the world for what it really is, would find a natural and sexual curiosity regarding the rugged, and wild Heathcliff; a man whom she has always been "warned" about and protected from since the time when they were all small children. Since Heathcliff's entrance in the lives of the Earnshaw's and the Linton's as a "rescued" child is enigmatic, nothing good has ever been concluded as to which could be Heathcliff's actual origin.
Aside from the possibility of Heathcliff being a gypsy illegitimate child, or a foundling, nothing else is known about him to those around him. Hence, Isabella's curiosity towards Heathcliff may have been cleverly used as a plot convention, but the consequences of it will enable Heathcliff's to comply with his avowal of revenge against those who mistreated and underestimated him, namely, Hindley Earnshaw and Edgar Linton. This being said, we can safely argue that Bronte's use of Isabella as the conduit through which Heathcliff will exact his revenge is a very clever convention but it is a very believable part of the novel. This is precisely because the character of Isabella is the most prone to fall for Heathcliff due to her upbringing and naivete about life.
Notice how Isabella is described. She has that "gold hair" that even Catherine felt a bit contemptuous about. Isabella also seems to have a unique and virginal beauty which prompts her family , in Catherine's words, "show her off".
She was at that time a charming young lady of eighteen; infantile in manners, though possessed of keen wit, keen feelings, and a keen temper, too, if irritated.
Yet, none of these things would move Heathcliff. He is disgusted by the mere thought of Isabella. Perhaps it is his feral nature which renders him unable to establish a healthy and solid human bond. It is also due to the abuse that he suffers at the hand of Hindley that makes him an evil and bitter man. Yet, it is the loss of Caroline to Edgar Linton what ultimately drives him to humilliate, degrade, and then abuse Isabella after he marries her. Heathcliff even dislikes his own son, procreated with Isabella. Therefore, it is clear that Isabella's story convincingly connects the series of events that will lead to Heathcliff's ultimate show of power.