Clara Copperfield is a great example of a "morally gray" character. She is loving towards David and obviously cares for him, and yet she always Murdstone to beat him. As a mother, she should stand up for her son. However, as a victim herself, readers can see that standing up for David will cause more hurt and pain.
Em'ly is another example. Having suffered so much from the class system her whole life, she deliberately tries to manipulate people in order to take control of her place in the system. She seeks marriage not for love, but for status, breaking Ham's heart along the way, and making many people unhappy. She herself admits that “I am not as good a girl as I ought to be.”
Uriah is the best example of what you have specifically stated in your question: a poor, and as he says "'umble" man who takes advantage of others. Uriah's interest in Agnes is only in what status he can gain through Agnes' father. Uriah maliciously interferes in the Strongs' marraige. In the end, he crosses from criminal mischief into bold criminal behavior, and he is imprisoned for bank robbery.