If The Mill on the Floss were a high school, then Lucy Deane would be the head cheerleader/class president/prom queen. And, to top it all off, Lucy would actually be a really nice and likable popular girl.
Lucy is a bit too good to be true. She’s beautiful, and very kind, and very friendly. Little cartoon birds probably help her get dressed in the morning. She genuinely cares about other people and she has an amazing capacity to forgive others. Lucy actually forgives Maggie after Maggie basically runs off with Lucy’s boyfriend/almost-fiancé.
Despite some obvious contrasts between them (their looks, their financial statuses), Lucy and Maggie have some significant things in common. Both are extremely compassionate individuals and both make an effort to think the best of those around them.
In fact, Lucy often seems to represent a sort of alternate universe to Maggie as an adult. We don’t mean this in the sense of something like a Star Trek episode (though that would be pretty funky); what we do mean is that Lucy is a lot of things Maggie might have ended up as, had circumstances been different.
So, let’s backtrack a bit with this. First off, it’s significant that Maggie as a child imagines herself as Lucy sometimes. Lucy is described as if she’s a little doll, with a "little round neck with her row of coral beads, her straight nose [...] her little clear eyebrows [...] to match her hazel eyes" (1.7.38). Maggie imagines herself as a "queen in Lucy’s form" (1.7.38), as if Lucy were a perfect and even fictional character that Maggie can pretend to be.
Lucy is cute and sweet and "perfect" in a way that the wild and awkward Maggie definitely is not. But we start seeing much closer similarities between the two after they are adults. When Maggie visits Lucy, she slowly grows accustomed to society and even begins to gain some social popularity. The frequently depressed Maggie seems like the polar opposite of the cheerful and naive Lucy in terms of personality. But, when Maggie is in a good mood, she is witty, enjoys teasing people, and can carry on a smart and fun conversation, a lot like Lucy.
Maggie is generally seen as "dark," in terms of looks and demeanor, or attitude, while Lucy's personality is "light," sunny optimism and happiness. But Lucy herself even points out that many of the differences between them is a matter of circumstance and not character:
"I’ve never been tried in that way," said Lucy. "I’ve always been so happy. I don’t know whether I could bear much trouble – I never had any but poor mamma’s death. You have been tried, Maggie; and I’m sure you feel for other people quite as much as I do." (6.2.22)
Lucy concedes that Maggie has experienced much suffering and is unhappy, but she refuses to admit that they are so very different in terms of essence or core character.
Lucy doesn’t just exist to act as a foil and a "what-if" possibility for Maggie, though. She definitely stands out as a positive character in a novel filled with depressed people. But Lucy is also an important alternative to Maggie, and her character makes a strong statement about the importance of circumstances in influencing characters' lives and personalities.