One of the most perplexing things about trying to fit the Les Mis characters into Hogwarts houses is that they’re so different across different iterations of the story. Sure, Valjean and Javert are pretty similar, and the Thenardiers are just funnier in the musical, but keep most of the same personality traits. But when it comes to the younger generation of characters, that’s not the case. The musical versions of Eponine, Marius, and Cosette are so different from their book counterparts that it’s almost impossible to decide on a definitive house for them.
Eponine: Gryffindor/Slytherin. In the musical, Eponine is a good-hearted and courageous young woman in a horrible situation. In the novel, she’s a desperate girl doing what it takes to survive, who is loyal only to Marius and will do anything for him. She is more involved with her father’s criminal activities than the musical suggests, and her actions at the barricade are less noble, since she brought Marius there in the first place hoping that they would both die together. Both versions love Marius, but in the novel, she loves Marius the way that Snape loves Lily: genuinely, one could even say selflessly, but with a mindset that is entirely fixated on him and wouldn’t care if the rest of the world burned to the ground. I would put the musical version of Eponine in Gryffindor and the book version in Slytherin; I guess it’s true that the two houses are two sides of the same coin.
Marius: Gryffindor? In the musical, Marius comes across as a brave, honorable young man fighting for a cause he believes in. In the novel … not so much. I think the best way to describe book Marius is “wishy-washy”. While his friends plan their revolution, Marius’ attention is easily diverted by Cosette, and soon, he barely cares about their ideals at all, only rejoining them at the barricade once he thinks he’s lost her. Likewise, he turns on Jean Valjean when the latter reveals himself to be an escaped convict, while in the musical, it’s only Valjean’s own insistence on leaving that separates him from Cosette. I’m not actually sure what house I’d put book Marius in. He’s not particularly loyal to anyone besides Cosette, and far from being ambitious or cunning, he’s easily tricked and can’t read people to save his life (for instance, he thinks Thenardier is a hero and Valjean a no-good criminal). Nor is he especially studious or intellectual, despite being a law student. Perhaps I would put him in Gryffindor, for the way he constantly speaks his mind and follows his heart, but it’s a vague, fickle sort of bravery.
Cosette: Hufflepuff/Gryffindor. Like Marius and Eponine, Cosette is very different in the musical and the book. The former doesn’t have much personality beyond being kind and sheltered, while the latter is outspoken and spirited, with a hidden longing for adventure – and for a sheltered girl who grew up in a convent, a secret romance with a young revolutionary must seem very adventurous indeed. She’s not involved in the more dangerous parts of the novel, but she speaks her mind and refuses to be ignored, silenced, or pushed into anything she doesn’t want, even by those she loves most. As Dumbledore once said, “it takes a great deal of courage to stand up to your enemies, but a great deal more to stand up to your friends.”