It's difficult to know what to think of Fantine in Les Miserables. She's a tragic figure who sings a fantastic song, but as she dies quite early in the musical, it also seems highly possible to dismiss her as a Victorian cliche. She is, to put it simply, the tragic fallen woman, who gets pregnant, turns to prostitution to support herself, and dies (assumedly) of consumption. Yet she also represents another cliche of Victorian literature, of the selfless, self-sacrificing, utterly devoted mother, and it is partly this combination of cliches, I think, that makes Fantine a powerful character.
Because let's be honest. The figure of the tragic prostitute, or the tragically fallen woman, and the figure of the sacrificing mother rarely meet as one figure. They belong in different realms -- both are likely to die, but one is a cautionary tale about female behavior and a romanticisation of downfall, and the other is a sentimental (if tragic) celebration of the extreme virtues of motherhood. The fact that Victor Hugo, and eventually the musical, combine these two common tropes of the Victorian novel allows them to present a character who pulls on the heartstrings in a reassuringly familiar way... but who also makes a bold and blunt commentary on the society that Hugo saw.
I only recently learned that Fantine's character was inspired by an actual incident witnessed by Victor Hugo in Paris. He witnessed a man harassing a prostitute in the street, and then, when the woman fought back, calling the police and insisting that she be arrested for her attack. Hugo interceded on the woman's behalf, but could not get the unfairness of the incident out of his mind, and his ponderings on the woman's history and the possibility of her having a child led him to create the character of Fantine. In some ways, this is a rather uncomfortable anecdote, not only for what it said about society at the time but also because Hugo felt the need to purify the woman into a completely selfless mother in his story to make her sympathetic. However, it also reveals the (plainly obvious, I think) attempts of both Hugo and the later musical at presenting a social critique, at trying to address real situations experienced by real women, and pointing out that the tragic fallen woman is not just a romantic cliche, but a reality that must be prevented.
Ultimately, I think Fantine succeeds as a character because, although she has little agency in the story and dies tragically early on, she is given a voice and a perspective of her own. She sings one of the musical's most powerful songs, and our hearts are with her the entire time she is onstage. From the time she appears until her death, Valjean is all but forgotten. This is her story, and it is a heartbreaking one, allowing her to be somewhat cliched, utterly sympathetic and compelling, a real-feeling character, and an excellent source of social commentary, all at the same time.