Cosette was the least of my worries for casting since she doesn't have many songs and her character could fit just about anypony. She has what I like to call the "Rapunzel" Personality, meaning all she really wants is a happy ending, to find her place in the world, and stop being treated like she's not able to handle it. It was initially down to Fluttershy and Cadence and since Fluttershy ultimately got Fantine, in the end Cadence got her daughter. Also, since Cosette and Marius are symbols of innocent love in war or hard times, I think it's perfect that we get the Princess of Love to play this role.

Cast:
Princess Cadance- Cosette
Applejack- Jean Valjean
Spike- Marius
Rarity- Éponine

P.S. I do not own my little pony or Les Misérables