If we were Buzzfeed, we'd title this section "Undeniable Proof that Cosette and Cinderella Are the Same Person."
Instead, we'll just say that Cosette's story bears a remarkable similarity to the famous princess. For starters, she has a wicked "stepmother" and two mean "step-sisters" who get preferential treatment from her "step-mother," Mme. Thénardier. The Thénardier family in general treats Cosette as a sort of slave laborer, even at the age of five. As the book tells us, "Cosette was made to run errands, scrub floors, sweep the yard and the pavement, wash the dishes and ever carry large burdens" (184.108.40.206).
But just like Cinderella, Cosette can dream. And like Cinderella, Cosette's dreams eventually come true. One night, for example, a kind stranger (Jean Valjean) shows up and gives her a very expensive doll that she's been coveting. At first, Cosette doesn't know what to do because she's so used to having a terrible life: "Cosette gazed at the miraculous doll with a kind of terror. Her face was still wet with tears, but her eyes, like the sky at dawn, were beginning to glow with a strange new brightness" (220.127.116.11). With good reason: her misfortune is about to change, now that Prince Charming—we mean, Jean Valjean—has come to take care of her.
When Cosette leaves the Thénardier house to live with Jean Valjean, she doesn't turn into a new person overnight. As the narrator tells us, "The sad fact was that at the age of eight her heart had been cold and untouched, not through any fault of hers or because she lacked the capacity to love, but because there had been no possibility of loving" (18.104.22.168). The book also informs us that for most of her young life, Cosette is ugly and misshapen, both in appearance and in spirit. Remind you of anyone? Like Valjean after prison, Cosette has been warped by her experience.
As time goes by, Cosette eventually feels secure in her new life and allows herself to enjoy it a little. One day, she even catches a glimpse of herself in the mirror and thinks she's become pretty, "and the discovery threw her into a strange state of perturbation" (22.214.171.124). But for all of these nice developments, the true transformation in Cosette happens when she realizes that she loves the handsome stranger who seems to love her back.
When she first notices, Marius, Cosette doesn't really know what romantic love is. She's been raised in a convent, after all, and no one has ever spoken to her about romance or anything like it. While she's busy growing up biologically, her mind doesn't seem to keep up with her. The narrator notes: "Thus did Cosette gradually grow into womanhood, beautiful and ardent, conscious of her beauty but ignorant of her love. And, for good measure, a coquette by reason of her innocence" (126.96.36.199).
In other words, Cosette has become beautiful enough to attract the attention of boys like Marius, but she's so naïve that she has no clue what's those funny feelings mean. It's only when Maris writes her a long and passionate love letter that Cosette comes to understand the meaning of love.
Brain snack: this is exactly how young women were supposed to be in the nineteenth century. Falling in love with someone before he fell in love with you was as bad for your reputation as having a bunch of nude Snapchats leaked would be today. (Actually, probably worse.) It was up for the guy to "awaken" your love by falling for you first. Good job living up to the double standards of your time, Cosette!
Once she understands, she knows that Marius is the one for her. As she tells herself, "She had loved him from the first. The fire had been damped and had died down, but, as she now knew, it had only burned the more deeply in her, and now it had burst again into flame and the flame filled her whole being" (188.8.131.52).
Her relationship with Marius so far has been on and off, but that's only because Cosette didn't realize how intense this kind of passion could be. And now that she knows, she wants to be with Marius for the rest of her life. Valjean? He might as well be chopped liver. But isn't that how it's supposed to be?