I have been unexpectedly finding myself spending a lot of brain-cycles on Montparnasse lately (possibly I have found my inner homicidal teenager at last) and I keep getting stuck on the Montparnasse/Eponine relationship.
A lot of the recent meta I've seen, and to some extent Hugo as well, implies that it's at least somewhat transactional, that Thenardier has essentially sold his daughter to Patron-Minette in exchange for favor - the "unofficial son-in-law" bit and so on - but the problem is, I can't figure out what Montparnasse would be getting out of it, in that case.
Thenardier is shown as being at best incompetent as a criminal and mostly peripheral to Patron-Minette, who are the premier criminals of Paris, and Montparnasse clearly doesn't give two shakes about Thenardier's criminal career, given he skips out on the kidnapping to canoodle with Eponine, so it's not about having Thenardier attached to the gang. Is he really so desperate to have a place to dip his wick that he's willing to be saddled with Thenardier deciding he's a son-in-law? He's the dandy of the sepulchre and she's canonically described as unattractive at that point; surely he can find someone to tumble who doesn't have an annoying father tagging along after.
So this leaves me with a couple thoughts on what the basis of their relationship might actually be, that line up more or less as:
1. Montparnasse is a lot of things, but he's also a teenage boy, and makes stupid decisions when it comes to sex: he really is just that desperate to dip his wick.
2. Hugo has no clue what he's talking about re: the relative power structures between Thenardier and Patron-Minette and we should look closer at what *happens* in those scenes rather than what Hugo tells us to figure out how they work.
3. Hugo has no clue and Eponine is actually objectively beautiful and a total catch by the standards of the underworld, fuck you and your equating virtue with beauty, Hugo.
4. Montparnasse wants Eponine for her own sake, for some reason or another, and M. Thenardier is only relevant to it to the extent that Montparnasse is willing to put up with him in order to get access to his daughter.
5. Montparnasse isn't actually interested in girls at all but needs to appear so because of Things, so having an arrangement with Thenardier via his daughter is a low-effort way to keep up appearances.
I was shoveling show this afternoon thinking about this, and I found myself intrigued by two directions those thoughts took me:
First, that Thenardier is clearly pretty terrible at the kind of crime Patron-Minette does - the violent kidnapping of Valjean is basically a farce from beginning to end, as is the planned robbery at the Rue Plumet - but that doesn't seem to be his specialty: his specialty, from what we can see, is running low-stakes con jobs, and in particular, writing letters. And suddenly it occurred to me that it makes sense that Thenardier's value to Patron-Minette is literacy: he reads and writes, and this gives him access to stuff that most people in that stratum don't have access to, and it's his reading and writing that he's depending on for his income, when we see him in Paris.
And Eponine reads and writes, too, and we see her being pitifully proud of her ability when she shows it off to Marius. Maybe what Montparnasse sees in Eponine is somebody who will provide him access to the world of literacy (...and as a bonus is also a not-terrible lay.)
But the other thing I was thinking about is that there's really only one character other than himself that Montparnasse shows any respect at all for, and that's Gavroche. It's pretty clear that Montparnasse knows very well that Gavroche is Eponine's sister and Thenardier's son even if Thenardier forgets sometimes, and he seems to have a sort of nostalgic brotherly relationship with him, to the extent that Montparnasse is capable of having an emotional relationship with anything other than his own wardrobe.
Bearing in mind that Montparnasse is within a couple of years of Eponine's age, it seems reasonable within the book's portrayal that Montparnasse has known the Thenardier kids since before he was the Dandy of the Sepulchre, that he first met them when he was a ragged street kid Gavroche's age, and I have visions of baby Montparnasse showing off for the new girls by teaching them the ways of the street and their first words of argot. He's not Thenardier's unofficial son-in-law because of the sexual relationship with Eponine, he's Thenardier's son-in-law because he's been a sort of a sibling to the Thenardier kids since he was small, and he's with Eponine because he and Eponine grew up together and fell into it naturally.
Weirdly, while Montparnasse is unquestionably a terrible, amoral person who is violent and probably sociopathic and exceptionally self-absorbed and certainly not relationship material, he does seem to have a sort of something resembling loyalty toward the people he considers his, even if he doesn't actually respect anyone but Gavroche: and that group seems to include basically just Patron-Minette and the Thenardiers. I feel like that's something I need to figure out before I can figure out Montparnasse as a character.