With a show like Les Mis, I don't think it's necessarily fair to rank characters' importance by the size of their roles onstage. I mean, the Bishop is arguably a major player in Valjean's arc, yet he's only onstage in the prologue. (And also at the very end in the movie, a change I really liked.)
With that, I'm sorry, but I really, really don't see how people keep arguing that Eponine is more important than Cosette. Cosette is not only the poster child (literally) for the entire show, but also the character who connects Jean Valjean to most of the rest of the story and contributes greatly to his character development. Valjean promises a dying Fantine he'll save her daughter and raise her, gets in trouble with the Thenardiers when he buys Cosette from them, finds out what it is to truly love when he adopts Cosette, later gets involved with Marius and the whole barricade plot because of Cosette, and then is able to die peacefully with Cosette by his side at the end of the show. For much of the narrative, Cosette is the glue holding things together and making different characters' arcs relevant to Valjean's story. The only two characters who aren't connected to Valjean's journey through Cosette are Javert and the Bishop, and that's because they're already involved from the very beginning. Eponine, meanwhile, really only has major relevance in the Marius arc, and the most important thing she does for the central narrative is deliver the letter to Valjean that prompts him to go help Marius out at the barricade (again...for Cosette). Cosette may not have a big, flashy solo like Eponine's, but her actual role in the story far surpasses her stagetime.
"Was uns befreit, das muss stärker sein als wir es sind." -Tanz der Vampire