I would suggest that it is the love of another, the redemptive feelings of sacrifice, that end up playing a major role in softening Eponine. Hugo constructs a character that straddles the reality in which she lives and the desire to rebel against it. Eponine is hardened by poverty and social degradation. Yet, her love and pure feelings for Marius play a major role in her softness. She wishes for the type of love that he shows to Cosette and her sacrifice is representative of the desire to love and to be loved. For Hugo, this is powerful enough to melt even the most hardened of souls:
What it came to was that in the heart of our society, as at present constituted, two unhappy mortals [Eponine and her sister] had been turned by extreme poverty into monsters at once depraved and innocent, drab creatures without name or age or sex, no longer capable of good or evil, deprived of all freedom, virtue, and responsibility; souls born yesterday and shriveled today like flowers dropped in the street which lie fading in the mud until a cartwheel comes to crush them.
Eponine's sacrifice is one in which she demonstrates how embodying the love of another and what it is we do love can seek to repel against the condition of the "cartwheel" that seeks to crush. For Eponine, her ability to love Marius is what drives her to take the bullet for him. Had she acquiesced to her streetwise hardening, she would have saved herself at his cost. Yet, she could not embrace this precisely because she embraced him. It is here where I think that one sees how love, the purest of kinds, is what enables Eponine to be softer than the world that molded her and how Hugo believes individuals can avoid taking the form of the world around them.