I think Cosette is braver in the face of the unknown (even as a child when Valjean shows up, she prefers the unknown adventure to the horrible life she knows) and Marius has courage in the face of the known (near-certain death on the barricade, deliberately chosen poverty and lack of family). That's why Cosette is the adventurer of the pair. And probably why Marius's stalkery ways don't bug her too much.
Fantine meta- I've had His Dark Materials and Eve on the brain lately and was looking at Fantine as a sort of felix culpa figure (appropriate in a book with such a strong sense of providence). Like Thenardier's evil actually results in a happy ending, Fantine's fall creates the situation in which Valjean's learning to love and completing his redemption are possible. If you peg her "fall" as the fall into poverty and prostitution, then in a sense Valjean is ironically redeemed by the consequences of his own sin (in firing her); if you peg it as trusting Tholomyes, then Cosette's existence (given the role she will play) is the good consequence. Which makes Fantine even more of a Madonna figure- her child is a savior!
Here day embraces night, and says: I will die with you and you will be reborn with me. From the heavy embrace of all desolations springs faith.
The real name of devotion is disinterestedness.