Speaking of "Castle on a Cloud", here's something I've always pondered...
Who is that "lady all in white"?
2. Some imaginary mother figure?
I actually accept both answers and can't decide which one I like better. Sorry for rambling but this is one of my favorite lines in the musical. Poor Cosette and Fantine!
It must be a bittersweet connection/parallel if she's really thinking about Fantine. There's basically no Cosette and Fantine interaction in the musical but I like the fact that they're somehow still connected at the beginning and the end of the musical. After Fantine gives her to Thenardier, mother and daughter suffers their own misery seemingly all alone, but the fact that Fantine suffers for Cosette and Cosette dreams of Fantine when she's sad always reminds me that though they're physically separated, they will belong to each other forever. And in their imagination, the other is always so perfect. I'm really happy that neither of them actually find out the truth of the other (maybe Cosette does, many years later, but I think she is able to handle everything by then). In the book, Fantine always believes that her Cosette is living happily and she is under this impression until her death. This is really so much better than actually seeing Cosette, abused, starved and cold, and knowing that she suffers all for nothing, before she dies! And for musical Cosette, Fantine will always be the nice, fairy-like white lady. I can't imagine how she feels if she discovers that her mother can do nothing to protect her and is actually on the edge of death.
Actually I think having Cosette dreaming of some imaginary mother figure is closer to the book, which elaborates more about how Fantine is thinking of Cosette than the other way around. First of all if this is ture, I don't blame Cosette at all. She's just 3 years old (not sure if in the musical she's aged up though) when Fantine leaves her. I can't expect a 3-year-old orphan to understand or remember everything. And my heart still aches for Cosette. She and Fantine have almost nothing; they only have each other in this world. To be fair though, she needs Fantine even more because she's just a vulnerable child who needs her mother's protection (off topic, but I slightly disapproves of Fantine leaving Cosette at the Thenardiers even though I know maybe she doesn't have any other choice. Again, I don't blame her, she's a victim after all). And now even this last posession is robbed of her. The Thenardiers are anything but parents to her. She surely has seen how they treat Éponine and Azelma, and all she wants is the love that resembles what's going on between the Thenardiers and their children. It doesn't need to be from a specific person because the desperate kid really doesn't know who she can turn to. Maybe (just maybe, I never want it to be true) she forgets Fantine, or feels disappointed...but anyway, there's no one in the real world she can think of that will possibly hold her and sing her a lullaby. And poor, naive Fantine is still thinking that Cosette will be alright. No seriously, nothing's alright.
OK I end up crying after typing this now I need to listen to something else to stop the tears...