As a child, Cosette becomes known in her neighborhood as "The Lark," but not for the reasons you might think (like freedom and beauty). In Cosette's case, "The Lark" refers to the fact that, like a lark, she always gets up earlier in the morning than everyone else. As the book says:
She was known locally as l'Alouette, the Lark. The village people, with instinctive symbolism, had thought it a suitable name for the apprehensive, trembling little creature, scarcely more than a bird, who was always first up in that house and out of doors before dawn. But this was a lark that never sang. (18.104.22.168)
This last line about how she's a bird that never sings helps symbolize how terribly oppressed she is, even as a young girl.