It's so sad to see how poorly dressed this girl is. She's shivering (this is repeatedly mentioned, too), and yet the word "string" is repeated to describe her clothes; she barely has anything to protect her from the weather. Hugo's focus on beauty when describing women and girls is uncomfortable in that it shows his own Thoughts on Women (valuing beauty above other traits), but like with Fantine, poverty deprived this girl of a chance to enjoy beauty (and in her case, from a very young age). Hugo is the one telling us that she used to be pretty, but she herself has some interest in appearances, commenting on Marius' beauty and looking at herself in the mirror. And as with Fantine, her lack of beauty is both physical and social. She, too, is missing teeth (whether because they were sold or she lost them due to illness or violence), making her look older and less appealing physically. At the same time, her lack of shame about not being properly clothed (she's only wearing a chemise and a petticoat, both of which were undergarments) makes her seem frighteningly shameless to contemporary readers and likely to Marius. Fantine lost her sense of shame, too, but not in a positive sense of gaining confidence; she felt no shame because she was ostracized to an extent where she felt she had nothing to lose socially. This girl has probably had a similar experience, since poverty is forcing her to go around with nothing more than undergarments on.
Her constant conversation combines the joy of childhood with the speech of the misérables. She's playful in a childlike way; she sings, for instance, humming in front of the mirror before she actually sings about poverty. The content of the latter is sad, but these ways in which she entertains herself serve as a reminder of her youth. To Hugo, children were inherently innocent, so her playfulness is a marker of that remaining innocence (even as he notes that poverty has taken much of that). At the same time, she rambles a lot, just like we saw Valjean do at the very beginning of the book and Champmathieu do during his trial. She repeats herself a lot ("as that is useless, as that is entirely useless, as that is absolutely useless") and narrates whatever comes to mind to Marius, likely because she has so few people that listen to her that she can't hold back now that someone's talking to her. Consequently, she jumps from one topic to the other. Marius has context for how she lost the letters, for example, but she shares her experiences with living under a bridge without any transition, as if she's desperate for someone to care about her experiences even if they can't understand them.
The differences in her and Marius' situations are also painful. Like him, she has some education (it's somewhat funny that she proves this by reading about Waterloo - I guess she can bond with Marius over Bonapartism, but since he's not as political now, it just suggests that he filled his room with books about Napoleon when obsessing over his father and never changed it because he then got distracted by pining). However, while Marius' education has kept him from the worst of poverty, hers can't protect her, likely because there are fewer opportunities available to women. She's not the only educated character who suffers regardless of their learning, though. The gravedigger Gribier was literate, too, but still had to work two jobs just to keep his family afloat. That being said, his education continued to provide in a way hers doesn't seem to (his day job involved writing), and he was more on the cusp of absolute poverty (if he lost one of those jobs, for instance) than actually living it.
The girl is generally much worse off than Marius. Most obviously, she's been homeless while Marius has not been. Marius, though, is still pretty poor:
"By dint of searching and ransacking his pockets, Marius had finally collected five francs sixteen sous. This was all he owned in the world for the moment. “At all events,” he thought, “there is my dinner for to-day, and to-morrow we will see.” He kept the sixteen sous, and handed the five francs to the young girl."
His charity guarantees that he'll be missing at least one meal, demonstrating that he's not that financially secure. Still, this girl has missed several (her words about breakfast suggest that she hasn't properly eaten for 2-3 days). She's hungry enough to eat the tough, moldy bread in Marius' room, too, which is heartbreaking to see.
