Éponine is the eldest daughter in the Thénardier family. The first thing we learn about her is that she has the raspy, unhealthy voice of an old woman when she's only sixteen years old (think Emma Stone, but way less cute), which is a big fat clue to the difficult life she's lived. When she meets Marius for the first time, she also shows that she's not one for formalities or politeness. She barges right into his room and asks him for money. When he gives her some, she exclaims:
Enough to stuff us for two days. You're a true nobleman, mister, and I tips my lid to you. Tripe and sausage and the tipple to wash it down for two whole blooming days. (220.127.116.11)
Besides showing us that she wasn't raised right, this ungrammatical slang—"tips," "tipple," "blooming," and so on"—gives us a clue to her background. Our French is a little rusty, but we're assuming that this "Eliza Doolittle before she meets Henry Higgins" translation is based on similar French slang, and is a way of showing Éponine's class status: low.
But just because she's not exactly high-falutin' doesn't mean she's an unregenerate reprobate like dear old dad. In fact, she protects Cosette and Jean Valjean's house from her father and his goons because she has such deep feelings for Marius. Selfish? Maybe. But the fact that she has the ability to feel deep feelings in the first place is a clue that life on the streets hasn't totally ruined her.
She's also got some backbone. When her father threatens her, she shows her bravery:
What do I care if my body's picked up in the street tomorrow morning, beaten to death by my own father – or found in a year's time in the ditches round Saint-Cloud or the Île des Cygnes, along with the garbage and the dead dogs? (18.104.22.168)
In other words, she's willing to die to save Cosette, even though girlfriend has good reason to hate her rival, just because it would make Marius happy. Selfish? Maybe. But it's still more than we would do. And then later, she does die to save Marius' life by jumping in front of a bullet. When Marius realizes what she's done, she uses her last words to tell him how she's always felt about him, saying, "You know, Monsieur Marius, I think I was a little bit in love with you" (22.214.171.124). You think?
The question we have is this: why? What's different about Éponine that makes her keep some tiny spark of goodness, when her parents are literally the scum of the earth? We don't know for sure; Hugo never uses any of his approximately 10 billion words to tell us. But we think it has something to do with Marius. Just like Valjean loves God (and Bishop Myriel) and uses that love as a guiding star for his whole life, Éponine seems to use her love of Marius as a guide to right action.
Sure, it's not as noble as loving God, and Éponine is never going to be as sweet and innocent as Cosette. But at least she's still capable of loving. And what is love? That big thing Hugo is trying to teach us: thinking about someone else for a change. In other words, empathy.