Liza is one of a long string of quiet, meek, passive, downtrodden women who inhabit Dostoevsky's novels. Through her, the Underground Man has the possibility of coming into touch with real humanity, but being unable to escape from his own ego, he needlessly and viciously insults Liza. However, she remains the morally superior person.
Through her own sufferings, which are real (instead of imagined, like the Underground Man's), she has developed a compassionate nature and an ability to love. We are deeply touched because the Underground Man is so callous in his feelings. Her life is one of simple expediency — she was sold into prostitution and feels the degradation and shame of her profession. Her feelings are intensified as a result of the Underground Man's depiction of the horrors of such a life. Yet, she seems to possess a simple faith and honesty that she can escape, that she won't be doomed to this type of life forever.
Even though Liza cannot "talk like a book," like the Underground Man, and even though she is simple and uneducated, she nevertheless possesses qualities which Dostoevsky considers more important than intellect or education. She is still in touch with basic humanity, she can understand the meaning of suffering, she can communicate with others, and even though the Underground Man has insulted and humiliated her, she forgives him because she intuitively perceives his torment and unhappy existence. Therefore, her simplicity and understanding, her sympathy and compassion place her as a morally superior person, in spite of her profession.