Rosamond Vincy competes with Dorothea as the major female character of the novel. Like Dorothea, she's very beautiful, but her beauty is of a different kind. Dorothea is very womanly and maternal, and is frequently compared to a saint. Rosamond, on the other hand, is always described as "infantine" – she's so blonde and fair that she looks childlike. The contrast between Dorothea and Rosamond is even played out in their names. While "Dorothea" means "gift of the gods" (see Dorothea's "Character Analysis" section for more on her spirituality), "Rosamond" means "rose of the world." Dorothea might seem like a saint, but Rosamond is thoroughly human. Her beauty isn't otherworldly like Dorothea's; it's completely of this world. So when some admiring Middlemarchers "called her an angel" (1.12.68), we have to assume that they're deceived: Rosamond's no angel.
But she's not some evil, plotting villainess, either; she's just kind of selfish. The narrator even defends her at one point, and assures us that Rosamond isn't interested in money for its own sake:
Think no unfair evil of her, pray: she had no wicked plots, nothing sordid or mercenary; in fact, she never thought of money except as something necessary which other people would always provide. (3.27.12)
So Rosamond isn't out to marry the richest person who proposes to her (because Lydgate's not the richest by a long shot); she just doesn't think about it at all. Thinking about money would be beneath her. The line, "she never thought of money" suggests that she just doesn't think at all. She assumes that someone – some vague "other people" – will always take care of her and provide enough money for her to have all the luxuries to which she's always been accustomed.
And how did Rosamond get this way? A lot is said at the beginning of the novel about Rosamond's education at Mrs. Lemon's school. The narrator describes it as
The chief school in the country, where the teaching included all that was demanded in the accomplished female – even to extras, such as the getting in and out of a carriage. Mrs Lemon herself had always held up Miss Vincy as an example: no pupil, she said, exceeded that young lady for mental acquisition and propriety of speech, while her musical execution was exceptional. (1.11.3)
So the "finishing school," as it's called (because it's where a young woman was sent to "finish" the education begun at home), gave Rosamond all the education that was considered necessary for an upper-class woman. The only examples of the kinds of things she learned include "getting in and out of a carriage," music, and "propriety of speech." All the things that Rosamond learns at school have to do with performing for other people. Her finishing school teaches her to be an actress in her everyday life.
And that's what she does. Rosamond rarely lets anyone know what she's really thinking or feeling, because her "mental life" is carried on below a perfectly polished surface. Even her eyes hide what she's really feeling, while still giving the impression of something else:
with eyes of heavenly blue, deep enough to hold the most exquisite meanings an ingenious beholder could put into them, and deep enough to hide the meanings of the owner if these should happen to be less exquisite. (1.12.68)
Rosamond "hides" her real "meanings" and allows "ingenious beholders" to project whatever they like onto her. This is why she's such a great performer. The narrator tells us, parenthetically, that Rosamond even acts her own personality:
(Every nerve and muscle in Rosamond was adjusted to the consciousness that she was being looked at. She was by nature an actress of parts that entered into her physique: she even acted her own character, and so well, that she did not know it to be precisely her own.) (1.12.31)
She is taught to act superficially so well at Mrs. Lemon's finishing school, that it's become an unconscious part of her own personality. The only time that she breaks down and acts naturally, we learn than she hasn't behaved so naturally since she was five years old:
At this moment she was as natural as she had ever been when she was five years old: she felt that her tears had risen, and it was no use to try to do anything else than let them stay like water on a blue flower or let them fall over her cheeks, even as they would. (3.31.61)
This is the first time we've seen Rosamond do anything – and we do mean anything – without calculating what effect it will have on the people around her. There's only one other scene in which this happens, and it's because of Dorothea's influence. Dorothea has generously gone to see Rosamond, despite her suspicion that Rosamond is having an extramarital affair with Will Ladislaw, and tries to help her save her marriage. Dorothea's simple sincerity has the effect of breaking down the shell of Rosamond's superficial act:
Rosamond, taken hold of by an emotion stronger than her own – hurried along in a new movement which gave all things some new, awful, undefined aspect – could find no words, but involuntarily she put her lips to Dorothea's forehead which was very near her, and then for a minute the two women clasped each other as if they had been in a shipwreck. (8.81.28)
Rosamond's entire character can be summed up as an artificial, but beautiful, product of a Victorian finishing school. The name "rose of the world" suits her. This woman might beautiful and delicate, but she's also worldly and superficial.