Hetty Merton is the young girl that Dorian has a brief dalliance with at the end of the novel. More significantly, Hetty represents to Dorian the hope of redemption, that he can actually love, and that he can somehow escape the fate that the bargain he made so many years ago seems to be pointing him towards. Note, for example, how he speaks of Hetty in Chapter 20 as he thinks about his various crimes and if his sins are catching up with him:

As he thought of Hetty Merton, he began to wonder if the portrait in the locked room had changed. Surely it was not still so horrible as it had been? Perhaps if his life became pure, he would be able to expel every sign of evil passion from the face. Perhaps the signs of evil had already gone away. He would go and look.

What is so key about Hetty Merton is the false hope that she offers Dorian. If his portrait displays the signs of his evil, he thinks, then surely his goodness will be displayed as well. If he is moral and good, if he is able to love purely and to treat Hetty Merton with that love in a way that is scrupulous and self-sacrificial, then maybe he can reverse the scarring of his soul that he witnesses in the portrait. However, it is when he sees that this is not the case that he determines to destroy the portrait, unwittingly destroying himself.